Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.
Why Do Aquarium Fish Swim Up and Down the Sides of the Tank?
Fish exhibit many behaviors that tell us how they are feeling, and glass surfing (also known as pacing) is one of them. This is when fish constantly swim up and down the sides of the aquarium glass. One reason they do this is stress. It could mean they aren’t happy in their environment, for one reason or another.
As aquarium keepers, it is up to us to examine the situation and determine the cause of their stress. There may be something we can change in their environment that will help them to calm down, such as improving their water conditions.
However, in many cases, the issue comes down to poor stocking decisions or tank size choices. This is why it is so important to research the needs of any fish you intend to stock before you even bring them home.
Stress is one of the biggest reasons aquarium fish die too soon. Watching out for signs of stress and taking action to reduce it can go a long way toward keeping happy, healthy fish. In this article, we’ll take a look at some of the common reasons fish glass surf and what you can do about it.
Poor Water Conditions
Maintaining clean, healthy tank water one of the most important things you can do to make sure fish remain stress-free. Waste chemicals build up in all tanks, and every fish keeper must take steps to reduce them. Of course, this means properly cycling your tank before you add any fish. Unfortunately, this is a step many novice fish keepers rush through or skip completely, and it may come back to haunt them.
Once an aquarium is up and running, it is vital to perform frequent water changes. This dilutes any waste chemicals, and if you learn how to use a gravel vacuum you can clean the gravel and change the water in one shot.
If your water parameters are healthy your fish are less likely to show signs of stress such as glass surfing. If you aren’t sure if your tank water is healthy it is easy enough to find out. You can take a sample to your local fish store and they can test it for you. I suggest having your tank water tested as well as the water straight from your source.
There are also inexpensive test kits you can use at home that will let you know how healthy your water is in a matter of minutes. There are a few options on the market, but I've always preferred the API Freshwater Master Test Kit. It’s very easy to use and it tests for pH, high range pH, nitrite, nitrate, and ammonia. The kit lasts a long time, even if you test daily.
Overstocking and Overfeeding
Overstocking is another problem that may lead to stress behaviors in your fish, and this issue is closely related to water quality. Along with your fish, there are millions of tiny microbes that live in your tank. That’s a good thing. They help to process the waste created by the fish, and because of them, it is easier to keep the tank water healthy.
This is why we cycle a tank—to allow the healthy microorganisms to increase in number. Ideally, the microbe colony in an aquarium will be able to break down the waste created by the fish. But when we have too many fish, things can get out of whack.
Put simply, more fish create more waste. When the bioload exceeds the ability of the microbe colonies to process the waste, the water quickly fouls. The same can happen with overfeeding, where there is too much uneaten food decaying in the tank. These conditions can lead to excess algae buildup or even a pest snail outbreak. More seriously, they can make your fish ill.
Your fish may also feel physically crowded in an overstocked tank, which can result in them pacing the glass. Fish need space to swim, and when a tank is crammed with other fish or decorations it can be stressful for them. Unlike in the wild, there is nowhere for them to go if they don't like their current location.
About Schooling Fish
So, to prevent your fish from glass surfing you’ll do everything you can to keep their water clean, so they don’t get feel stressed and get sick. And, you’ll avoid overstocking so they don’t feel overcrowded. The second issue is very important for semi-aggressive fish like gouramis, angelfish, and bettas. Too many other fish in their territory can make them downright ornery.
But some fish prefer to have other fish around, especially other fish that look like them. Some, like guppies, platies, and mollies, are best kept in groups with two females to every one male. This gives them the social interactions they need without undue stress on any one fish.
For other fish, more is better. These fish live in huge schools in the wild, with numbers ranging in the dozens, up to the hundreds or even thousands. This natural behavior is very important to consider when stocking fish in the home aquarium.
This is yet another reason why it is so important to research the fish you intend to purchase. Schooling fish need to be in groups of six or more, and that includes obvious species such as neon tetras and not-so-obvious ones such as corydoras catfish.
While they can make excellent community fish, schooling fish that don’t live in large enough schools tend to exhibit unnatural, stressed behaviors such as glass surfing or, worse, occasional aggression. I always recommend a minimum of six, but more is better if you have a large enough tank.
Incompatible Tank Mates
Just as it is important to make sure your fish are living with the right tank mates, it is equally important to ensure they aren’t living with the wrong ones. A bullied fish is under constant stress, and you may find them pacing the glass or hiding all the time.
Once again this comes down to understanding the fish you intend to stock. For example, if you intend to have tank mates with a betta fish you need to put some effort into researching your best options, and how to do it correctly. In some cases, an aggressive betta may cause stress for other fish in the tank. In other cases, the betta may be picked on, and he’ll be the one exhibiting signs of stress.
There are less extreme examples. If you keep a pair of male gouramis one may bully the others. A pair of angelfish may or may not get along, and one may lash out at the other, or at other tank mates. Fin nippers like tiger barbs may stress angelfish.
Once again, doing the research necessary beforehand can help you understand how your fish might interact, and if they are a good match.
Finally, the size of the tank itself is a big reason a fish may be stressed and unhappy in its environment. I can’t tell you how many messages I get from people saying they have a betta in a two-gallon tank and he seems unhappy, or they have a completely inappropriate array of fish in a five-gallon tank.
Without enough room to swim and live a fish will be stressed. Any animal confined to too small of living space is going to want out.
To make matters worse, small tanks are harder to keep clean, and it is harder to maintain healthy water quality and proper temperatures. If you intend to have more than a few fish these problems are exacerbated even further.
All of the issues listed above are magnified in small tanks. I always recommend at least a five-gallon tank for a single betta fish and a ten-gallon tank for tropical fish. Sometimes this alone is all it takes to solve problems with stress behaviors.
What to Do About Glass Surfing
Occasional glass surfing isn’t anything to worry about. There are even some instances where pacing the glass is normal and expected, such as when a fish is new to its environment after you perform a water change or other major tank maintenance, or when people are standing near the tank and the fish may expect food.
But maybe you’ve gone through the situations above and don’t believe any of them apply to your situation. Your water perimeters are good, your tank is the appropriate size and not overstocked, your schooling fish seem happy and are kept in the appropriate numbers and there are no aggressive fish in the tank. What now?
It’s important to remember that fish act in all kinds of unexpected ways in an aquarium. You can continue to experiment with ways to relieve what you believe may be causing the stress, but don’t stress yourself out trying to get to the bottom of it. Then you’ll be the one climbing the walls!
Make the best decisions you can for the fish under your care, and learn along the way. That’s all any of us can do.
This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.
Eric Dockett (author) from USA on August 31, 2020:
@Patricia - I don't know much about breeding fish. My guess is they can move in as soon as they are big enough not to get eaten. You'll want to do some specific research on this though. Good luck.
patricia on August 30, 2020:
Erick otos have been moved in my 20L tank and are doing great but i got a new issue otos are with my adult Gold Barbs in which the Gold Barbs had fry the fry are now in the 10 gal tank Grow out tank off and on or periodiclly the fry (which are now about 10 weeks old ) and looking like the adulds there about a inch ong but a couple of them do the glass surffing i just done a tank change and water par. are perfect can the fry go into the big tank yet is tht why there surffing if not when can they join there parents in the 20L
Rafael on August 28, 2020:
1.5-2 Gallons is easily the best option for bettas alone. If you want to add more fish in a tank with them i would suggest bringing the tank size up to around 10 gallons or more but be careful because they are territorial and aggressive.
Chuck on August 22, 2020:
I raise bettas and I can tell you its a myth they need a tank of 5 gallons. I don't recommend keeping them in jars but bettas tend to stay relatively static . They don't like moving water and 5 gallons is a waste of tank. Give them a gallon, no aeration and regular water changes and they are very happy.
Eric Dockett (author) from USA on August 05, 2020:
@Fishie lover - That's a tad high. It wouldn't hurt to bring it down a little, but I suspect there is another issue somewhere. Water parameters are good? Current and temp are good?
Fishie lover on August 03, 2020:
Hi I have about a 20 L or 4.3 gal and have a betta fish. He sometimes kind of kind of put his head against the glass and kind of swims into it. There are no other fish in there. The only think I think is wrong is the pH is about 7.5 ok? Or do i need it lower?
Eric Dockett (author) from USA on July 24, 2020:
@Mia - Sometimes fish just do strange things. Hopefully that is all it is. Good luck!
Mia Groenewald on July 23, 2020:
Hi, ill do a water test tonight but currently with all my tests the ph is good (i cant remember the value but its in the green/good zone)
Maybe its because i removed their cave, although i never really saw them utilise it. I did move the drifwood cave from one ent to the other end of the tank, maybe they're being picky hahaha .
The tetras are 5x lemon & 5x Xray's they all shoal together. I know my 3 harlequins are too little, im actually judt waiting for the store to get more stock, so definitely adding more .
It's so weird seeing this because other than them acting so funny, there arent any fatalities, so im holding thumbs that they just being weirdos hahaha. But ill do a water test again and see
Eric Dockett (author) from USA on July 23, 2020:
Hi Mia - I can only take guesses of course, but here are a few thoughts:
About the loaches: I'm wondering if you changed something about their hiding spot when you changed the tank and either they can't get to it anymore or can't find a new spot they like. I am also thinking, because they are scaleless, that the additions to the tank may be affecting them more than the other fish.You said you thought the water quality is okay, but did you test it? Maybe there has been a pH swing with all the plant matter you've added? Again, just guesses.
About the rasboras and tetras: I've noticed many small fish do weird things when they aren't in big enough schools. Sometimes they go a little crazy with water changes too. An influx of fresh water simulates rainfall and the start of the rainy season, which may kick in their spawning instincts.
I don't know how much any of this helps. It's hard to guess. I hope you get it figured out!
Mia Groenewald on July 22, 2020:
Hi, I have 2 questions. 1: In my 50L tank I've been noticing my kuli loaches a lot. Like they were already pretty confident and coming out even when the light was on (my other tank's loaches hide all the time) Anyway, I've done an upgrade on the aquascape and added more plants and some almond leaves and occasionally add some of the water I boiled the almond, alder, banana leaves, cinnamon sticks and cones in. SO I've noticed the loaches would swim up and down the one corner of the tank every single day, maybe its been 2 months now of them doing that. No fatalities and the other fish are super chilled. Not sure if something is up, like if they are trying to swim upstream to a better place, so that means I need to redo the aquascape; or if they are just loving life and playing. I have seen a stressed loach before, swimming with his head out the water all frantic, ---this is not what they look like, they barely touch the surface.
in my 30/40l tank, I have a bunch of fish and the 2 types I've noticed doing the following was harlequins and lemon tetras. Ok, so I've had the harlequins for 6months and they swim like crazy like they love it when I do water changes and they would swim against the current, now I've read that they sometimes glass surf because they see their reflection and is trying to school with more of their kind. I and waiting for my aquarium shop to stock more harlequins to see if this theory is correct and I just need more in my school. But what I've noticed with the Tetras, that I've only had for 2 months, is that I spotted 2 of them swimming nose up, against the current...I've never seen something like that and all the fish seem fine. Good and healthy weight and not really shy and just happy and eating well. I can't find anything other than swim bladder disease but I know it can't be that.
both the tanks are semi heavily planted and the smaller invertebrates are surviving like my shrimp and so, so I can't imagine that there's a parasite or disease or distress or whack water parameters going on...
Eric Dockett (author) from USA on July 20, 2020:
@patricia - Good to hear it! It is smart to quarantine new fish for at least two weeks. Good luck!
patricia or pat from Glendale, arizona on July 19, 2020:
ty Eric for your response Ottos doing better today feeding Rapashy Soilent Green and added Cattopa leaves everyone in the take is doing great no more glass surffing by the way when can i put them in the 20 L tank and out of quarantime
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Eric Dockett (author) from USA on July 18, 2020:
@Pat - Are they moving to a different tank eventually, I assume? I'd keep trying with the algae wafers and see how they adapt to the new tank.
patricia or pat from Glendale, arizona on July 17, 2020:
i just got 4 otos about 5 days ago i have them in my quarantine tank i see two of them like to do this glass surfing i tryed feeding them algae wafer and they wouldnt eat them so i tryed rapashy and they go for it great but one still doing the glass surfing it make's me stressed to see them stressed i want to helo them but dont know what to do, water parameters are perfect they have plants ect. what more can i do
Eric Dockett (author) from USA on July 17, 2020:
@Josh - Sorry it took so long do respond. Has he calmed down at all? The tank is a little small for him but I assume he is a juvenile. (?)
Josh on July 14, 2020:
Hi I bought a Texas Cichlid fish and I put him in a 30 gallon tank and he keeps surfing the glass sometimes then he’ll go back to normal like he’ll swim hide a little then swim but sometimes he just surfs idk if it’s the tank size or my ph
Eric Dockett (author) from USA on May 22, 2020:
Hi Blaire. Wow, I really don't know much about them, expect that this is one of the most interesting fish I ever heard of someone keeping. I am going to guess that they *probably* should be in schools and that the tank is *probably* too small for him, but again I am just guessing. The little info I can find on them seems to support those guesses. I suspect if you treat it like any other tetra species you might have good results. Good luck!
Blaire on May 20, 2020:
Hi Eric! About two years ago I acquired a Mexican tetra fish (the surface form, not the cave form). He was in a lab but jumped out of the tank and the professor gave him to me because he didn't know which tank he came from (this is important later). Anyway, he is a hearty fish and I haven't had any problems with him. He does, however, often pace the glass. I'm not sure if this is from being kept in a lab or something else. He is currently in a 5 gallon tank. Do you think he could be lonely? If so, what kind of fish should I try to pair him with? I say "him" but the other issue is that I don't know the gender of the fish (hence why I acquired "him"). There is virtually nothing about keeping a surface form Mexican tetra online that I have seen. I realize the information here has gaps, but I would appreciate any insight you may have! Thank you!!
Booboo on May 15, 2020:
I don't think my fish are stressed out, I believe they are playing.
Gazelle on May 04, 2020:
I changed my betta female koi to a larger tank at first she was excited and happy a few days later she started to glass surf non stop i tired everything i think she was happier in the old tank
Eric Dockett (author) from USA on April 17, 2020:
@Kierra - What size tank is he in and what kind of fish is in there with him? A 5-inch goldfish would ideally have a 20-gallon tank all to himself.
Kierra on April 15, 2020:
I’m taking care of my friends fish,( he is a 5 inch goldfish) and he keeps on pacing the glass I’m wondering if it is because he is REALLY STRESSED or if the tank is two small
( he also lives with a much smaller fish about half an inch long) PLEASE let me know, that’s kind
Eric Dockett (author) from USA on February 18, 2020:
@Yesika - It sounds like you are trying your best. The more you can do to improve their conditions the better their chances will be. The reasons it may be glass surfing are listed in this article. Take a look around the tank and see if there is something that may be causing stress. (Air stone, maybe?) Also, I believe the tetra glofish are bred from blackskirt tetras, which are schooling fish. It may be that the fish feels stress being in such a small school. Hard to say. But I wouldn't add any more fish to that small tank.
Good luck. I hope he calms down.
Yesika Guzman from USA on February 17, 2020:
Hello! I have three tetra glofish and this is my first time caring for fish. I’ve taken care of other animals (bunnies, hamsters, dogs, chickens and parakeets) but never fish so I really don’t know...
I recieved them from my dad who got them from his boss who were going to throw them out. They used the fishes as decor and kept them in a wide-ish wine bottle. After the party they were moved to a 5”x5” vase and brought to my home.
With my budget I was able to ony afford a 3 gallon crescent tank that came with a filter, air pump and led airstone. I quickly went to buy some glofish rocks to put so the airstone stays down but I’ve noticed that one of them sometimes glass surfs... Sometimes it stops and joins the other two that swim around the bottom.
They also don’t seem to be eating much as well... what I have as of now food wise is the Tetra baby shrimp and the Tetra tropical flakes. They don’t eat none of those and I don’t know what else to do.
Sadly I wasn’t really able to get them to adjust properly to water or had the way or understood that the water should be checked. All I was given was some baby shrimp and conditioner.
Im hoping to get a water heater, some fake plants, and a cave in two days.. but as of now what can I do to help my surfing fish???
I want to give them a better chance at living. We weren’t prepared to have fish but we also didn’t want them being thrown out after being decor for a few hours...
Eric Dockett (author) from USA on February 07, 2020:
Hi Jamie. If you just got the a few hours ago you have to give them some time to settle in. You didn't mention if you cycled your tank and tested your water. Those are two important steps.
Jamie on February 06, 2020:
Hello. I just recently got my two pregnant guppies a few hours ago. I did all of my research, and even set the tank up before taking them home. The temperatures are correct. The lamp is on. Already fed them. They have water movement and air. I have the conditioner for the water i use and even a stress coat. I dont get why they do it, especially one of them that does it constantly. Is something that i did wrong? They have a cave and plants, and a 5.5 gallon tank. Itd mean a lot if you could help, thanks.
Eric Dockett (author) from USA on January 24, 2020:
@Diana - Did you perform a water change and test the water before adding the fish? Appearance alone can't determine whether a tank is cycled. You need to test the water, especially if you used old tank water. (In the future, you don't need to do this. If anything, a small piece of old filter fabric or sponge added to the new filter would do as well if not better.)
He could just be adjusting. If the water tests okay and temp, water flow, etc aren't causing stress you might try making small changes to decorations. I would test that water and see where it stands, though. Good luck!
Diana on January 23, 2020:
Hi, i recently got betta. I cycled the aquarium beforehand with water from another already cycled tank and put some starting bacteria in to help the proces. The water is crystal clear and everything is fine- i just put him in today and he cant stop pacing. I have a black background so i thought its reflection but he isnt flaring or anything. Could it be the new environment ? and if so how long does it usually takes for him to adjust ?
Eric Dockett (author) from USA on December 06, 2019:
Hi Michael- One technique fishkeepers use with livebearers is to stock at least two females for every male. That helps alleviate the situation you are describing, where one female is harassed relentlessly. It can cause stress and even death if left unchecked. I don't know if that is an option for you, but it could help.
Unfortunately, I don't have a lot of experience breeding fish. I suggest talking with someone who does so you can find out the best way to manage her while she is pregnant for the safety of her and the fry.
Michael Chavar on December 05, 2019:
My swordtail is pregnant and already had one fish, so we put her in the breeding box. She seemed too stressed and didn't like the small amount of space to continue having her babies, so after 2 days we put her back into the tank. We have 2 other fish, a platy, and a molly. The platy is the one who got her pregnant. When we put her back in, however, the platy keep chasing her around. I heard it means they are trying to mate, but I guess the platy doesn't understand shes pregnant. The female trys to hide and relax, but then mostly surfs the side of the glass when this happens. Could you help me out on what to do? It has now been 5 days.
Eric Dockett (author) from USA on November 29, 2019:
@H3II-0 - They ought to settle down in a day or two if everything else is in good shape.
H3ll-0 on November 28, 2019:
How long would fishes in a new tank display such behavior?
I have 9 Rosy barbs that were in a 10 gallons (they hatched there, their eggs came with the plants) than I moved to a 55 gallons this weekend. They are it's only population with a few snails.
I used Stability to make sure the new waste intake isn't too much, I have Prime in case of and monitor the parameter. So far, no ammonia, no nitrite and just a little nitrate.
Eric Dockett (author) from USA on August 11, 2019:
@IsabelaK - Unfortunately I can't say whether or not your fish will die overnight. I can say you have a few things working against you. The tank is a little small, and you are cycling it with the fish in it. It is a good idea to get a freshwater testing kit so you know you water parameters. Otherwise you are just guessing if or when the tank is cycled. I'm not sure what the shop person meant by the readings not being reliable. You can certainly see if you have high levels of ammonia. Poor water conditions could account for the glass surfing.
Female bettas are typically best kept in "sororities" of five or more fish. I know you are doing the best you can with what fell into your lap, so let's hope they continue to get along. I would not add any more fish to that tank.
Good luck. I think the most important thing, as with any tank, is to know your water parameters so you know if the water is safe or not. Otherwise, it is hard to guess what is going on.
IsabelaK on August 10, 2019:
I have a bit of a query. I was given 2 betta females by someone who bought them by accident and I had to set up a tank pretty quickly to save them from a flower vase of all things (they were in time-out for nipping guppy tails). The setup I got is a 30L tank (about all I can afford money and space-wise) with all the necessary accessories - heater, filter, light, long leafy plants for them to hide between and so on. Since the whole thing was time-sensitive (again, living in a vase with no filter/heater) there was no time to properly establish the tank. The person at the aquarists store in town advised me to use a bacteria supplement to help with it. It's been about 5 days now. The ladies are peaceful and have no issue with one another, and are actually very human-friendly, but I've noticed that one of them glass surfs a lot for the last 2 days. She didn't use to do it before - she was actually pretty chill compared to the other one - all the way until I put some filter sponge on the filter output pipe to weaken the flow a bit (it was a little strong before and made them struggle and visibly annoyed them, now it still makes the water move but isn't pushing them around). From the moment I switched the filter back on, the spazzy girl became chill and started to calmly explore the foliage (and finally lost the stress stripes), while the calm one took to glass surfing? I haven't checked the water levels yet, as the tank is not even a week old and the person at the shop advised me to wait for a bit more with that as the readings won't be very reliable as the tank establishes itself... Should I be concerned about my girl's behavior or should I give her some time to get used to the tank and new conditions? Apart from making the filter output weaker nothing changed. I spend some time every day staring at them (pretty fish teehee) and haven't noticed any physical changes so far (part form stripes disappearing).
I quess, as a novice to the whole fishy business I don't know a lot yet and just need some reassurance that they won't die on me overnight:0
Eric Dockett (author) from USA on January 12, 2019:
@Sandy - It's hard to know why your molly is glass surfing without knowing more about the tank. Tank size, tankmates, etc? Livebearers like mollys are usually best kept in trios with one male and 2 females, but of course as you realize that opens up the possibility of little mollies.
@Comet - Bala sharks are schooling fish that grow to up to a foot in length and should be kept in groups of six or more. To keep bala sharks you need a very, very large tank - much bigger than your 20 gallon. Lack of a school is possibly the reason for the glass surfing. Also, your bala shark will eventually get big enough to eat your neons.
You may wish to try to rehome the bala shark and consider more appropriate fish for your 20 gallon. I know that's probably not what you wanted to hear, but a bala shark simply isn't appropriate for a 20-gallon tank.
Comet on January 12, 2019:
So my balashark has been glass surfing for all I can remember. I had him with my Betta fish for a while (maybe being bullied?). They had episodes where they switched sides of the divider but one day Coral my Betta fish died. I moved the balashark to my 20gal tank and him alone. No tank mates until today. He glass surfed when he had plenty of room and space. The decor I have is three live plants and a driftwood piece that takes up less than half my tank. Jump my balashark has six tank mates now, all neons. So I’m wondering what’s wrong with my balashark. If anyone can help thanks.
Sandy on January 08, 2019:
I only have 1 male molly who is swimming up and down the glass. Should I get another male for company? I had a molly baby problem before and don't want to go thru that again. My other fish are danios and glofish and my water was checked and fine.