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Top 10 Mistakes New Fish Hobbyists Make

Marie is a lover of everything about and inside of aquariums. Among other friendly creatures, she has a turtle that she adores.


Watch Out for These Mistakes

1. Not Treating Tap Water Properly (Or at All)

Tap water contains chlorine, which is safe for humans, but a death sentence for aquatic creatures. Most of us learned this early on; either tap water must sit out for 24 hours so the chlorine evaporates, or it has to be treated with one of the numerous dechlorinators out there to get rid of the chlorine instantly.

But chlorine isn’t the only danger in tap. There can be ammonia and nitrites in tap water (again, safe—just barely—for humans), which is why more experienced hobbyists use such dechlorinators as Prime, a treatment that detoxifies these poisonous chemicals in addition to removing chlorine.

There may be other dangers that cannot be treated by liquid in a bottle. For example, extremely high or extremely low pH, heavy metals, and high nitrates can exist in drinking water, the last being the worst offender (as partial water changes would be pointless). If these levels are dangerous, most would suggest a R.O. system that hooks up to your pipes in order to filter out these pollutants.

2. Not Cycling New Tanks Before Adding Fish

This mistake is way too common, and it ties in with the fact that a lot of people don’t know about the nitrogen cycle. We were taught we must perform partial water changes in order to remove fish waste before it compromises the fish’s health, but it isn’t that simple. The truth is fish are constantly living in their own filth; they urinate and defecate every day. So how do we get away with performing partial water changes? How do fish last so long before then? Answer: The Nitrogen Cycle.

A natural process built over time, beneficial bacteria eat toxic ammonia produced by fish waste, turning ammonia into nitrites, another toxin (albeit less toxic). Next, the bacteria eat all nitrites, turning them into nitrates, the least harmful and only toxic at 30-40ppm. So a chain follows in this natural system, turning the toxic presence into a safe one, until nitrates (the end result) get too high, which is the point of performing water changes; we do it to reduce nitrates.

However, you don’t have this beneficial bacteria when you set up a new tank. In order to build this system, you must have water, a running filter, and ammonia to start the process. It takes anywhere from 4—8 weeks for the cycle to complete, and then it will be safe for fish.

Of course, a lot of people don’t like the idea of having a running aquarium for 1-2 months without anything living in it. Some will resort to putting hardy fish in to help regulate ammonia and the cycle, but I discourage this. You will lose those fish eventually during the procedure, and it’s just cruel.

If you are too impatient to wait, you can buy beneficial bacteria in a bottle and reduce the 4-8 weeks to a week.

3. Too Few Water Changes and No Gravel Vacuuming

It doesn’t matter how good your filtration is, the end result is always the same: nitrates. Unless you have nitrate-absorbing filter media, only partial water changes with low to no nitrates will reduce them significantly, giving your fish a breath of fresh air—so to speak. No matter how light the biological load is, you can’t get away with monthly water changes and expect healthy fish. Nitrate poisoning will kill and add even more nitrates to the water (from the carcasses), in addition to causing mini cycles.

You can get away with biweekly water changes, but the amount changed will have to be a big percentage, and this depends on water and waste ratio. It’s better to perform small weekly water changes than large biweekly ones, but it can be done with a light bio load.

If you’re going to siphon out the water, why not vacuum the substrate while you’re at it? If you don’t regularly vacuum the substrate, waste builds up over time, causing nitrates to rise faster than they should.

4. 100% Water Changes

I don’t know much about bowls, but when changing out an aquarium, you want to avoid performing more than a 50% change within 24 hours, unless it’s an emergency (like exposing bleach in the tank). Some say huge changes are a don’t because you lose beneficial bacteria, but these grow on the surfaces of objects in the water; only a tiny percent are in the water itself.

The real reason you don’t want to perform huge water changes is that you may put your fish in pH shock. PH is the level of acidity (soft water) or alkalinity (hard water—minerals) in the water, and while most ranges are tolerable for fish, fluctuating pH can kill.

Water straight out of the tap often changes in pH after being exposed to air for 24 hours. This isn’t always the case, but it is the usual. My tap comes out at 7.4. 24 hours later in a cup, it’s 8.4. It was this in reverse in my last home.

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PH shock will stress and potentially kill fish, especially if their immune system wasn't up to par to begin with. When changing pH, it must be gradual. It’s often recommended to not change your fish’s pH more than 0.3 within 24 hours, unless they are hardy fish, then you may get away with a 0.5 change. This is why small partial changes are better than big ones.

If you must perform a huge water change (if it can wait), let all the water you plan to put in the tank sit out for 24 hours.

5. Removing All Filter Media at Once and Rinsing With Tap

Most beneficial bacteria live in the filter, specifically on the filter’s media (floss, charcoal, sponge, and ceramic rings). The filter may “polish” your water, keeping it clear and odorless using charcoal, but the true purpose of the filter is to keep your fish alive through the nitrogen cycle. I hate how Hang On the Back filter (HOB) companies encourage the practice of completely switching out the filter media for the sake of convenience. You are doing harm to your tank every time you replace that cartridge with a dry one because beneficial bacteria have to be kept wet.

Instead of replacing the entire cartridge, just rinse it in order to knock off built-up debris in the floss encasement (so water can properly flow through), and just replace the active carbon if you wish to continue chemical filtration (although many consider charcoal to be a gimmick and unnecessary). While charcoal should be replaced every two weeks, the filter floss can go a month without replacing—in most cases. But if you are rinsing filter media, don’t use tap water. Chlorine kills bacteria, so instead, rinse or scrub the floss gently in a bucket of aquarium/dechlorinated water.

6. Overstocking Your Tank

When people set up a new aquarium, they get excited and want the tank to be filled with activity, but they can, without knowing it, go overboard. Just because you can squeeze 40 guppies in a 20-gallon doesn’t mean you should. Remember, the fish have to live in the tank, not you. Would you want to live in a small house with a hundred people? With piles of excrement in every room, with no escape from the wretched, poisonous air? That’s what nitrate poisoning is, and in order to avoid this with your overstocked tank, you’ll need to perform large water changes several times a week. Just avoid overstocking; return fish if you can, or give them away to a friend or on craigslist.

For opinions on stocking ideas, ask on fish forums. You can also go to, and it will give you a rough estimate on stocking ideas compared to space, fish compatibility, and filtration.

7. Overfeeding Fish

A lot of fish deaths are either the result of ammonia/nitrite/nitrate poisoning or overfeeding. Unlike cats and dogs and mammals in general, fish will keep eating even if their stomachs explode. In the wild, they don’t know the meaning of being full, so you can’t trust their behavior. Fish will always act hungry. They say when you feed fish, they should only eat as much as they can in 2-3 minutes (and this is for one meal a day). So if there is still food in the tank after 2-3 minutes, remove it immediately.

8. Lack of Research on Fish Needs and Compatibility

So many people buy fish or other aquatic creatures on a whim in stores when they need to consider the following:

  • How big do they get and what is the minimum size tank for this species?
  • What temperatures do they prefer?
  • What pH do they prefer?
  • Do they need to be in groups? If so, how many?
  • Are they aggressive or peaceful?

It’s not enough to know if they are freshwater or saltwater. All species have different needs. Do your research before ever adding fish to a community tank.

9. Using the Aquarium as a Nightlight

Fish need daytime and nighttime too. They sleep like any other animal, and that darkness helps them register when it’s time for bed. Fish can sleep with the lights on, but it’s healthy for them to be in the dark. As tempting as it is to just leave the light on, remember to shut it off when you go to bed. I leave my aquarium lights on for 12 hours a day. If you’re forgetful like me, buy an aquarium light timer you can get at any aquarium store so they can turn on and off on schedule.

10. Dumping Meds in at the First Sign of Trouble

Fish medicine is a last resort. When fish become ill, it is almost always because the water parameters are in bad shape, meaning there is either ammonia or nitrite in the water, or nitrates are at a dangerous level. When you suspect your fish have fungus, bacterial infections, or parasites, first check your water parameters to make sure everything is where it should be.

When you suspect fish are ill, immediately perform a partial water change, no matter what; it’s the safest move you can make when treating your fish. Freshwater aquarium salt can also be added to help with healing, but don’t overdose. Scaleless fish and invertebrates are sensitive to high salinity, so do not add more than one tbsp per five gallons to avoid harming them. Remember that salt doesn’t evaporate; only partial water changes will remove it, so don’t keep adding salt if you’re not changing out the water. Often water changes and aquarium salt alone treat the early stages of any illness.

The reason you should use meds as a last resort are a) it’s very difficult diagnosing fish, as a lot of different ailments have similar symptoms; the water change and salt treatment is broad and effective, b) some meds can hurt other species of fish (such as melafix and labyrinth fish, or copper ingredients and invertebrates), and c) meds can destroy your beneficial bacteria, which will cause ammonia and nitrites, and that will probably lead to more fish deaths than the illness itself.

For me, the last resort is after five days of performing water changes and adding salt appropriately and the symptoms continue to worsen. That or when the fish stops eating, then I’ll purchase medicine, as some illnesses are so advanced that you need something stronger. Make sure you know what’s ailing your fish before giving them medication. Research each medicine you are considering and see what their side effects are: will they hurt invertebrates? Will they kill beneficial bacteria?

If there is just one fish showing signs of illness, take it out of the tank and treat it with medicine in a hospital tank.


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.

© 2014 mariekbloch


Mark on August 02, 2020:

We have hard water in our aquarium set up but our PH is fine is that bad or can we put the fish in a

Steve from Chingford London on June 13, 2020:

Aha. I agree with most of what the author says. I have to say though, some extra advice could be useful regarding Vaccuming substrate, especially when there may be many delicate plants involved and in the way. Once,.I was a maticulous vaccumer. And the plants did become a massive problem and how to avoid disturbing them seemed a pain.. I eventually developed the 'chopstick' method. Fixing a the end of the hose, stir gently around the plant released some muck from the sand/gravel and the hose positioned about 2 inches above did the rest.

This might be helpful to..those of you feeling faint hearted about lunging in with the mighty gravel Vac.

Hope this was useful.

Jmart70 on February 22, 2020:


I was reading the article about about cleaning the filter on fish pond and how removing all the build up can be harmful to the fish. I am hoping I can get some help here. I just had this happen

Courtney on June 03, 2019:

Hi Guys! I recently bought a betta fish from a petstore (I had planned on buying one online but I instantly fell in love). My concern is when I purchased him, he had cloudy eyes. I read online that this can be caused by a multitude of reasons-- from popeye to cataracts to dirty water. My fish tank at home is cycled, and he has been here one day. Should I try aquarium salt, or should I try something else? It may be a birth defect, and that's fine, but if it's something else I want to get the medicine that he needs.

Adam on June 01, 2019:

I agree with most of these, but if you use proper filtration and plants, water changes can be extremely minimal to non existent. I personally heavily plant my tanks, use a Hamburg mat filter, and a fluidized sand filter. My parameters are always perfect, and my water changes consist of taking a few cups of water to top off my cars and bowls, and then topping off my tank once per month, which is around a 1% change.

Jon K on April 22, 2019:

I've had a large tank for several years and have made almost every one of these mistakes! I learned over time, and my fish are pretty happy at the moment, but I lost a lot of them along the way. Really wish I'd read an article like this when starting out!

Don Pratt from United States on February 04, 2019:

Very helpful article!

April on January 12, 2019:

Can I get a fish after a water change?

toxic on December 27, 2018:

i have a fish the dwarf gourami and after a water change he was weird he swim on his side and was not as active he used

to be

mariekbloch (author) on October 24, 2018:

PH should be close to 7, but stability is more important than a fixed number. Put dechlorinating liquid in the new water, wait 5 minutes, then perform the water change. Algae isn't a bad thing. It adds oxygen to the water. One betta in a 3 gallon is fine, but with frogs I'd upgrade to a 10 gallon so its less crowded.

LCM5479 on October 22, 2018:

Thank you for that informative article. We have a 3 gallon with an African Dwarf frog and a beta and have an algae problem...all over the rocks, live plants, decorations. I was considering using a waste away product. When doing a water change, is it safe to use both a dechlorinator and the waste away at the same time? Also, what should the PH be for a tank this size?

Thanks in advance!

mariekbloch (author) on August 25, 2018:

It was running for a year, but were you occasionally adding ammonia or fish food so the nitrogin cycle was being fed? If not, then they died of ammonia. And if you did, but never did water changes, then they surely died of nitrate poisoning. It's not enough to just run a pump. Without adding food to decay in the tank and performing occasional partial water changes, then it doesn't matter how long it runs: it will kill fish. Look up how to perform a fishless cycle.

Clint on August 22, 2018:

I have had a tank set up with no fish all new stuff new filters pumps ect for over a year with no fish i was shure the watter would be safe by now and i got some new fish and they all died can someone tell me a way to figure out why and what i need to do to insure this doesnt hallen agian?

mariekbloch (author) on June 07, 2018:

Taylor, I talk about this based on experience. I had a fish that I overfed. I'd watch him eat every single flake, and I realize now it was too much. Over the course of a week, his belly swelled up and his scales stuck out, from swim bladder disease, which can be caused by consitpation due to overfeeding. Overfeeding is still bad even if the fish doesnt eat it, as you said, it can lead to an ammonia spike. But fish can overeat. I've never seen a healthy fish turn down food.

Taylor on June 06, 2018:

Fish do NOT eat until their stomachs explode. That an old wives tale. The reason is that the extra food decays on the bottom and causes ammonia. Also fish do NOT sleep like we do although it is beneficial to have a schedule it is not like you would have people be led to believe.

mariekbloch (author) on May 30, 2018:

That's right.

Sathish on April 06, 2018:

YES this site tell truth and believable.very nice this is very much useful to everyone who loves aquarium

LeBron James on March 12, 2018:

this are amazing facts about aquarium

Giovanni Carlo from Purok Saranay Tawagan Norte Labangan Zamboang del sur on December 24, 2017:

These are all true the common mistakes is adding fish to newly water without aging it they thought that clean water is safe they did not know that it lethal to a fish from the gases in it that will penetrate the fish body just like divers disease called bend.

Verity on December 08, 2017:

I love this article, thank you. I get advice from the shop and had perfect water levels for my new fish, as well as having pond running for about 5 weeks. All of these rules are highly regarded! I have a question. I keep an eye on Ph levels, and they seem ideal for goldfish as the tap water tests about 7 or 7.2 water seem to keep it low which is great. I have Ph down in case there are any troubles. My question is that I wonder if ammonia levels can be a good indicator of nitrate levels? My ammonia was a little above 0 and when I added fish it was 0, so it seems to be cycling well. I want to become familiar with my tank. For example, if I get mussels I will be looking at water hardness, or calcium levels. But, is it necessary to test nitrite and nitrate, if I have no problems with ammonia? I ask as I can get free tests at the shop anyway, and I don't want to pay to do it from home if it's unnecessary. Nitrite and nitrate were testing at 0 at the shop. I have white clouds and Ph is staying good, water change due in a few days. Thank you!

Jennell on November 04, 2017:

Is it safe to usesoft water in my new aquairium?

SteveD0391 on July 23, 2017:

@Roxy this sounds like a bacterial bloom, its when bacterial colonies increase so much that they become visible, this can happen in newly cycled tanks and is part of the nitrogen cycle. The bloom should solve itself once the cycle stabilises and you'll be able to introduce fish to the tank slowly.

Roxy on July 14, 2017:

No fish in the tank, used Cycle a day ago as directed by pet shop and still cloudy. What gives? No drift wood, rinsed everything properly before filling, filter is working, heater is working. Am I ever going to be able to put fish in it?

Stephanie G on July 06, 2017:

i just put 2 small koi and 2 goldfish in a 75 gallon tank

its been about a week the water is really dirty do i do i 20% water change

i have been feeding them 2 to 3 times a day is that okay?

Allison on May 14, 2017:

i just move 1 5" goldfish into a new tank. I used the older hang on the back filter in my new tank and took the new filter and put it in the old tank (with no fish). Can I move the "new" hang on the back filter into the new tank. it has been hanging and working in the old tank with new fish for about a week.

Juan on April 07, 2017:

hello, i have a cobra guppy and two platies and two shrimp in a five gallon tank i change the water 25% once every week and i was wondering if thats good for the fish

laurira on April 01, 2017:

Most of your advice is sound, but... I think people put too much stock in the whole pH shock thing. I've had a lot of mistakes along the way and tbh I've moved 3x and had to break down and put my tanks back up. Logistics in moving large tanks beING what they are, Ive always ended up having to replace 50- 75% of the water. I haven't lost a fish yet and this last one, it was 30 degrees out and snowing.

Healthy fish areally more resilient than people give them credit for. And that brings up temp fluctuations. While I'd never put a fish that is in a bag at 65 degrees into an 8p degree tank, the idea that a slight fluctuation will shock and kill it is naive at best (slight meaning 5 degrees give or take). I've had acclimating fish get accidentally dumped too soon by an eager kiddo and no losses -- unless I bought them from a chain pet store (lessons learned, let me tell you)

Also, the thing about fish eating til they explode with food is just plain myth. They will stop when they are full and to believe otherwise is just plain silly. You don't over feed them because doing so leads to a filthy tank.

As far as water changes go, if you have a plant free tank, yes, weekly is mandatory. If you plant your tanks-- which will always lead to happier and healthier fish, the nitrate is almost always next to nothing and, while you might have gross substrate from a cosmetic standpoint, as long as you have nothing catastrophic happen (such as when my 3 year old dumped a large can of food in and no amount of gravel vaccing could save my java moss), the tank becomes very forgiving-- healthy and happy fish even if it over grows a bit and becomes asthetically unpleasant.

And third, regarding illness... I used to be in the aquarium salt camp/water change camp. I used to jump up and down and scream from the heavens about it. Then I learned that the only thing I need to do to cure ick was to slowly turn my temp up to 88 degrees (over a period of about 2 days-- I keep my tanks coolish at around 68).

The ONLY store in my area I will ever purchase fish from quarantines all fish for a month before selling a single new stock fish AND medicates any batches with sick fish. Meds aren't bad, IF you know what youre doing.

The key is knowing that and knowing when the rest of your tank is on key.

Some might say "we'll you've just gotten lucky." Trust me, I've had enough stupid mistakes over the years to fill a book. It's know in when you're being stupid and when you're not.

mariekbloch (author) on January 25, 2017:

pH control does more harm than good. Prime is excellent, as it detoxifies bad chemicals in your tank, declorinates the water, and adds a healthy slime coat to fish. Also, if there are harmful chemicals in your water, like ammonia, Prime is only a temporary fix. It merely makes it tolerable for fish until the nitrogen cycle kicks in.

Zach on December 26, 2016:

What chemicals should I add to my tap water to make it fish safe? I know you need de-cholrinator, but anything else? Like ph control?

mariekbloch (author) on October 27, 2016:

I've never had Mollies before, but the tank size does seem too small. 10 gallon would be nice for them, and you could add one or two Mollies. I feed fish once a day with a pinch of flakes, but since you only have two fish, maybe 4 flakes per day? Thanks for replying.

Elton on October 26, 2016:

Hi. I fave two molly fishes about 2 to 3 inches, and I feed them in a 20cm x 10cm small fish tank. May I ask that is it ok for my fishes, and how many food should I feed them everyday, and also how many time should I feed them a day? Thank you.

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