Glass Surfing: Why Do Fish Swim Up and Down the Glass?

Updated on August 20, 2019
EricDockett profile image

Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.

Betta fish often swim up and down the glass when they are stressed in their tanks.
Betta fish often swim up and down the glass when they are stressed in their tanks. | Source

Why Do Aquarium Fish Swim Up and Down the Sides of the Tank?

Fish exhibit many behaviors that clue us in to 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.

Schooling fish like tetras need to be kept in the appropriate numbers to avoid stress.
Schooling fish like tetras need to be kept in the appropriate numbers to avoid stress. | Source

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 makes 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 they 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.

Watch for bullying behaviors in species such as the dwarf gourami.
Watch for bullying behaviors in species such as the dwarf gourami. | Source

Tank Size

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 a 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 your 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. You 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.


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    • EricDockett profile imageAUTHOR

      Eric Dockett 

      5 weeks ago from USA

      @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.

    • profile image


      5 weeks ago

      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

    • EricDockett profile imageAUTHOR

      Eric Dockett 

      8 months ago from USA

      @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.

    • profile image


      8 months ago

      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.

    • profile image


      8 months ago

      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.


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