21 Reasons Why Aquarium Fish Die

So your fish mysteriously died last night and you don’t know why. It could be one or several of the 21 reasons below.

Think about something you may have done that you didn’t do before. That could be the cause or maybe it’s just that you’re not taking care of your aquarium. In any case, please look over the reasons below to find out if it was from something you did, and also things to keep in mind for the health of your fish.

I start with the obvious things and then move on to the not-so-obvious.

Poor or no filtration: Too much ammonia or nitrites

If your filter is not large enough to cycle the water in your tank or if you’re not cleaning the filter regularly, you will have problems with excess ammonia and/or nitrites in your water. Too much will kill your fish eventually.
Make sure you have the right size filter. Clean it regularly (except for the bio media) and perform regular water quality checks for ammonia and nitrites. I go over how to test your water here.

Sudden change in water temperature

Maybe the heater malfunctions or the power goes out, and suddenly the water temperature drops drastically. The opposite can occur if the heater malfunctions and the water gets too hot.

Either situation is not good for your fish. A sudden change down can severely stress the fish if they are the type of fish that can’t survive in colder temps. A sudden change up means less oxygen in their water, and most fish need oxygen in the water to survive. Fish don’t have body heat like humans, so it’s up to us to regulate that for them.

Make sure you have a backup heater in case one malfunctions. You can replace it right away. It’s also a good idea to have a separate thermometer to read the temps. Don’t rely on the heater-thermometer. I explain the dangers of a malfunctioning heater here.

Sudden change in water pH

You may bring home some new fish in a bag and then dump them into your fish tank thinking everything is alright, and then within days, they die. It could be that the pH was much different than they are used to. You must acclimate new fish to your fish tank first for water temperature and pH. Find out at the place of purchase what the water parameters are so you can determine if the fish are a good fit for you.

Untreated tap water

City tap water contains chlorine and chloramine to keep the bacteria for human consumption. Excess chlorine can kill the good bacteria needed for proper filtration and eventually kill the fish, so when you do water changes, you need to use a water conditioner specifically for chlorine and preferably one that treats chloramine as well.

Another thing to be careful of is when you go to clean your filter and other aquarium equipment. Make sure to use the aquarium water to clean, not tap water.

I go over how to make tap water safe here.

No aquarium heater

Not using an aquarium heater is fine for certain species of cold water fish, but most tropical fish can only survive in higher water temperatures in the 70-85 degree range.

Lack of oxygen in the water

As the water temperature gets hotter and hotter, less oxygen will be present for the fish to breathe. This can happen if an aquarium heater malfunctions, the fish tank is located near a window with direct sunlight, or there may be a heat wave with no air conditioning.

Certain species of labyrinth-breathing fish like betta fish may be able to survive for a while, but this could be a serious issue. Have a separate thermometer that constantly monitors the water temperature so you can address the issue right away.

The problem could be not enough water flow or poor filtration. It’s also a good idea to keep the water circulating with an air pump or powerhead to create surface agitation and water circulation. Make sure your filter is up to the task, and you regularly change out the mechanical and chemical filters. Here are three reasons you need a water pump.

Didn’t cycle the new fish tank long enough

For every new fish tank, you have to establish a nitrogen cycle. If there are not enough good bacteria in the biological media, your filter can’t absorb the harmful ammonia and nitrites. This is known as ‘new tank syndrome.’ It’s important to establish the nitrogen cycle and test the water until the ammonia and nitrite level is zero. Constantly test the water. Here’s how to cycle your fish tank fast.

Overfeeding the fish

You should only feed your fish what they can consume in two minutes. Overeating is bad for us and for fish as well. They get fat, their organs start to suffer, and disease may occur. Another byproduct of overfeeding is that the uneaten food waste settles in the substrate, and as it decomposes, it causes harmful ammonia to be released.

Once a day, feeding is usually sufficient, but two smaller feedings could work too. The only exception is if your fish are herbivores meaning they are strictly vegetarian.

Fish tank is too small

If the fish tank is too small, there is less water for the fish, which means waste can build up much faster and foul the water. When selecting fish tanks, bigger is usually better because it’s easier to keep the water clean (unless you overcrowd the aquarium).

Certain fish like to have lots of room with hiding places. They may get stressed and die if they are in a small tank. Do a little research first and find out about the fish you plan on getting. What are their habits and preferences? Are they schooling fish? I talk about tropical fish compatibility here.

Unsuitable fish food

There is a lot of cheap fish food out there filled with phosphate and other junk. Feeding your fish a steady diet of this is like you eating fast food all the time. It’s just not good for them. At least if you are going the fish flake route, make sure it’s decent quality food with very little filler.

Most fish, especially carnivorous fish, like a variety of foods. Frozen bloodworms, krill, or shrimp are great. Spirulina pellets are great for herbivores and omnivores. Do some research on your fish and find a good diet for them. Here’s a pretty good selection of fish food on Amazon.

Disease and parasites

Watch your fish closely if they start to show signs of weakness like hanging out at the bottom or top of the tank a lot more, turning to the side or just not swimming like they used to, or rubbing themselves up against decorations and substrate. You may notice white specks on their skin, or the fins are starting to look raggedy, it could be Ich or Fin Rot. If they become bloated, it could be signs of Dropsy, a more serious disease not easily treated.

Make sure to have some treatment on hand, like aquarium salt and Melafix for parasitic infections, so you can treat them immediately. If possible, you should set up a quarantine tank to separate the diseased fish from the others, so it doesn’t spread. If you don’t have a quarantine tank, do a water change and add some aquarium salt and other treatments depending on the disease.

Improper water change

Never do a 100% water change and remove the fish. You should do a partial water change every other week with the fish in the tank. A 100% water change is too much of a shock for the fish, especially if it’s not treated and the temperature and pH are all off.

A more appropriate percentage would be about 25% because the water parameters won’t change near as much, and it’s not so much of a shock for your fish. Before adding the water back into the tank, ensure the water temperature is close to the tank’s temperature. Also, ensure the new water is treated for chlorine if it’s city tap water.

Waiting too long between water changes

Even with good filtration, you must do partial water changes regularly. The reason is that while your filter system should remove the ammonia and nitrite toxins, you still have nitrate to deal with. Now nitrates are not as harmful as the other toxins but could weaken your fish over time if levels are too high. If you have a lot of live plants, it’s not so much of a problem because healthy plants absorb much of the excess nitrates.

Another reason is that over time, essential minerals in the water will settle and become ineffective at regulating pH. Over time, your tank could crash, and the pH level could drop dramatically. That could be deadly for your fish.

You should change the water with partial water changes every other week or so. Some get away with once a month, but it depends on how many fish you keep with your size tank.

Fish already diseased or sick

You may have purchased new fish from a pet store and not realized that the fish were already diseased. A lot of these places have poor conditions for the fish. They will overbreed them, feed them cheap food or use tons of antibiotics to keep the fish alive. With all their other duties, the store clerks just don’t have time to properly take care of them. Certain store chains use a centralized filtration system on all of their aquariums so the disease can spread more easily to the other tanks.

Go to places that specialize in tropical fish. You may have to spend a little more, but it’s worth knowing the fish are of higher quality and have been cared for better.

Using detergents to clean a fish tank

Using detergents like Clorox or even dish soap is a big no-no. Any residue left over will kill your fish pretty fast. Instead, you’ll want to use something like vinegar or bar keepers friend to clean the tank itself if there is a buildup of calcium and other stains. Here’s how to clean your fish tank safely.

Overcrowding your fish tank

Too many fish, too little room, too much fish waste, too many ammonia spikes, and too little oxygen equals stressed-out fish. That makes sense, right? When it comes to fish tanks, bigger is usually better because it’s much easier to manage the water quality. Like any other pet, fish need to have room to roam and establish territories. If you don’t provide that, it will be stressful for them.

So what exactly is overcrowding? Well, it’s not so cut and dried. It kind of depends on the type of fish you have, the size of the surface area, and how large your fish will grow. Schooling fish like rasboras or barbs will require more room. Goldfish tend to produce more waste, so you need to take that into consideration. Bettas are territorial and fight, so the fewer, the better. The point is to research recommended stockings for each type of fish. Equally important to know is how big they will get.

Another factor is the size of the surface area on your fish tank. The larger the surface area of your fish tank, the more oxygen exchange will occur. That’s a good thing. A tall, narrow tank has a much smaller surface area than a wide rectangular one, so you can’t stock as many fish in a tall tank.

Using air fresheners

The jury is still out on whether or not this is true, but to be on the safe side, ensure your fish tank is covered if you spray air fresheners. Instead of spraying, you might try an oil-based freshener or candles or plugins well away from the aquarium.

Using flea spray around the fish tank

Flea spray is super toxic to fish, so you probably want to do this outside. If you do spray inside, you should tape some plastic around the top of the fish tank first to make sure none penetrates. You could also spray in another area or level of your home. Ant and roach spray can also kill your fish, so be wary of any potentially toxic things you may use in your home and not even think about.

Noise pollution

Loud stereo speakers or loud equipment next to your fish tank or even slapping the aquarium real hard can cause severe stress to your fish. This is because fish have a lateral line sensory system that helps them detect changes in water pressure. Any sudden change can be a shock to their system. Any loud noises terrify fish and cause stress.

Using hand soap or sanitizer

You may not even think about it, but if you use a hand sanitizer, perfume, mosquito repellent, or suntan lotion, dip your hands in the fish tank. Many products like these can be lethal to fish, so you want to ensure that you are not dipping your hands into the tank after applying anything. Use a fish net, tongs, or some other instrument to do what you must do. Only use tools specific to the aquarium that haven’t been in the dishwasher.

Excessive cigarette or cigar smoke

Some fish are more sensitive than others, so if you’re a heavy smoker and your space is not well ventilated, you may want to keep a hardy breed of fish like goldfish or danios. They’re not so sensitive to smoke toxins. It never hurts to have a cover over the tank as well.

Final Thoughts:

It’s advisable to test your water pretty regularly for toxins like ammonia, nitrites, and nitrates to see where you’re at with water quality. Also, monitor the water temperature.

Sometimes your water could be fantastic with no nitrites or ammonia, but you’re fish are dying anyway. At that point, you’ll have to do some investigation and think of things that you may have done recently, like spraying or you changed the food or the water or putting your hands in the tank. If you changed something, that would be the first clue as to what may have caused the deaths. You may not even be aware of some of the things that can be fatal to fish.

Let me know below if you’re guilty of any of the above.

When it comes to fishkeeping, what is your single biggest challenge or frustration? Leave a comment below.

Marian Haaz
Marian is a cat adoption expert. She has rescued and adopted several cats herself and helped find homes for dozens of stray cats and dogs.
Photo of author

Leave a Comment