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10 Reasons Tropical Fish Die in a Tank and How to Prevent It

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

Find out why beautiful aquarium fish die and what you can do about it!

Find out why beautiful aquarium fish die and what you can do about it!

When Fish Die for No Reason

Managing a tropical fish tank is a lot of fun, and very rewarding. It can also be incredibly frustrating when your fish keep dying.

You find a tank you like, put it all together, get it running and follow all the advice given to you by the guy at the pet store. It’s a beautiful moment when you add your first fish and watch them puttering around in their new home.

It’s not so great the next morning when you find your pretty new fish lifeless and stuck to the filter intake. What happened?

It is always heartbreaking when fish die, especially when it seems like they die for no reason. You so badly want to do things the right way, and you don’t understand what went wrong. It is normal to feel discouraged, but remember that the most important thing is to learn something, so that your future fish are better off.

Though it may not seem like it, there is usually a good explanation for a fish dying. Whenever you lose a fish it is an opportunity to revisit your tank management and care practices, and that’s true whether you are a newbie or an experienced fish keeper.

In this article, you’ll read some of the common reasons tropical fish die too soon. It is intended to help you diagnose and correct the problems that are leading to the death of your fish, whether your tank is new or established.

10 Reasons Fish Die in a Tank

This is a long article, so here is a quick list of topics it will discuss. You can scroll down to learn more about each.

  1. Stress: Stress is the number-one killer of aquarium fish.
  2. Lack of Tank Preparation: Failure to cycle a new tank can cause problems.
  3. Inappropriate Aquarium Size: Choosing a tank that’s too small for its inhabitants will lead to trouble.
  4. Incompatible Tankmates: Not all fish get along.
  5. Poor Water Conditions: When the water goes bad, fish start to die.
  6. Overfeeding: This one is easy to get wrong, but so important to get right.
  7. Bad Tank Management Practices: Perform water changes, clean the gravel and manage algae if you want your fish to thrive.
  8. Disease: Fish get sick sometimes, but there are some things we can do to prevent it.
  9. Dumb Moves: We all make mistakes. Go easy on yourself!
  10. Issues Beyond Your Control: Sometimes it isn’t your fault that your fish died.
Gouramis can be territorial and bully others of their kind.

Gouramis can be territorial and bully others of their kind.

1. Stress

I listed stress a first here. However, just about everything else in this article causes stress for your fish, in one way or another. It is the most common cause of death for aquarium fish, and managing stress is a key part of keeping your fish healthy.

Signs Your Fish Are Stressed

As you’ll see, there are many factors that impact the stress level of your aquarium inhabitants. But how can you know if your fish are stressed? Sometimes you can’t know, but there are some typical signs you can look out for.

  • Glass Surfing: This is when your fish repeatedly and sometimes frantically swims up and down the sides of the tank. Something is wrong, and the fish would go somewhere else if it could, but it is stuck in the tank. Maybe the tank is too small or too crowded, the water quality is bad, or there is another fish in the tank that makes it uneasy.
  • Excessive Hiding: Fish hide when they are scared or uncomfortable, or just want a little peace and quiet. That’s okay sometimes, but if you have a fish that is constantly hiding it may be because it is being bullied by another fish in the tank. This can be caused by territorial issues (poor stocking choices) or an undersized tank. Be aware that is normal for certain types of fish to hide, such as catfish, loaches, and plecos. This is why you need to research the behavior and care requirements for every fish you stock, so you know when something isn’t right.
  • Weight Loss: Obviously you aren’t going to physically weigh your fish, but you can visually tell if a fish is becoming emaciated. Sometimes this can happen even if they are eating. I’ve seen this caused by stress from bullying alone.
  • Illness: Stress can often lead to illness. I'll cover this in more detail in another section, but if you notice your fish are constantly sick there is a good chance they are stressed. Stress compromises their immune systems and makes them more susceptible to disease. If a fish looks ill you need to try and figure out why ASAP, especially since it can potentially transmit disease to other tank inhabitants.

If you see any of these signs in one or more of your fish it is time to find out why and make a change. The rest of this article can help you figure that out.

2. Lack of Tank Preparation

The situation described at the beginning of this article, where all or many of your fish die, is common in new tanks that haven’t been prepared for fish the right way. In really bad situations the deaths could occur overnight, but typically they will happen over a few days or a week. This is often because a tank hasn’t been cycled correctly.

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Read More From Pethelpful

Fish aren’t the only organisms that live in aquariums. A healthy tank also has a vibrant (microscopic) bacteria colony. They live all over the tank but are most concentrated in the gravel and filter. These microorganisms help to break down the waste in the tank and keep the water healthy.

They have a symbiotic relationship with the fish. Without the fish (and their food), the bacteria colony declines. Without the bacteria, the water quickly becomes unsuitable for the fish. The two must be kept in balance for a healthy tank.

In a brand-new tank, there is no microbe colony yet. You have to “cycle” the tank to start to build it. There are several ways to do this, and you may have received instructions with your new tank. I suggest researching the nitrogen cycle and the different methods of cycling a new aquarium in order to decide how you’d like to proceed. Only once the tank is cycled should fish be added.

Adding too many fish at a time can also cause problems as well, even in a properly cycled tank. The microbes need time to grow with the number of fish (and other inhabitants) in your tank. Therefore, if you intend to have 20 fish in your tank eventually, add four or five a week over several weeks and give the bacteria colony time to keep up.

Many species of pleco become require very large aquariums.

Many species of pleco become require very large aquariums.

3. Inappropriate Aquarium Size

There are two ways to go about choosing an aquarium. The first is to decide on a tank based on the space you have, and then stock it with appropriate fish.

The second is to decide which fish you want, research their requirements, and then get the right size tank for their needs.

Unfortunately, what some novice aquarium owners do is a combination of both. They get a tank based on the space they have and then buy inappropriate fish without understanding their needs. This can lead to big problems in your tank, and the loss of some beautiful fish.

One problem is the often-repeated “one-inch-of fish-per-gallon” rule. This means each gallon of water in your tank can safely house one inch of adult fish. So, for example, if you have a ten-gallon tank, you can have five two-inch fish.

This works okay for little fish like neons and guppies, but it is soon clear that, as a rule, it really makes no sense. Would you keep a twenty-inch fish in a twenty-gallon tank? Would you keep one eight-inch and two one-inch fish in a ten-gallon? Would you keep a pair of fish each over two feet long in a 55-gallon tank?

I hope the answer in all cases is, “Good grief, no!” You may think this is an exaggeration to prove a point, but there are some really big fish you can buy from a typical pet store. You purchase them as small juveniles, and they grow into monsters. Always do your research before you purchase!

Whether you buy a tank first or decide on fish first, either way, it is important to understand the needs of the fish you intend to stock. A tank that’s too small for its inhabitants can cause stress, promote disease, pollute quickly and lead to the premature death of your fish. This is a problem often seen when bettas are kept in tiny aquariums, but it can happen in any tank.

4. Incompatible Tankmates

Along with choosing the wrong size tank, choosing tankmates that don’t get along can be a big problem for beginners. Some fish are territorial and will chase any other fish in their claimed area. Some fish are aggressive and should only be kept with other fish that can defend themselves. Some fish are fine with other types of fish, but can potentially have issues with others of their own kind.

This is why it is so important to research a fish before purchase, so you are sure they are a fit for your tank. Sometimes it can be hard to know, and it is worth it to take your time. The situation is made worse by pet stores and their employees who sometimes give erroneous advice on care needs and compatibility.

I learned this lesson the hard way myself. Years ago I purchased a pair of juvenile green spotted puffers from a pet store that told me they were freshwater community fish. In actuality, I found out, they are ferocious little killing machines with complex care requirements. Thankfully I got them out of my pretty community tank before they demolished anyone.

The positive of that experience was that I learned a lot about puffers. I re-homed one and got a tank for the other where I kept him alone in a brackish setup. But it could have been much worse.

So, was it the store’s fault for giving me bad info? Kind of, but really it was my own stupid fault. I should have done my homework before buying.

In aquarium care, we sometimes learn tough lessons. One of the reasons I write these articles is to help you avoid doing some of the dumb things I have done!

Fragile tropical fish require clean, healthy water to thrive.

Fragile tropical fish require clean, healthy water to thrive.

5. Poor Water Conditions

Bad water equals dead fish. There are a lot of significant takeaways from this article, but that’s a big one. Keeping the water healthy for your aquarium inhabitants is one of the most important things you can do to avoid premature death for your fish.

It is smart to have a testing kit so you know where you stand with your waster parameters. These kits typically measure ammonia, nitrates, nitrate, and pH. I recommend the API Freshwater Master Kit as I’ve always found it really easy to use and more accurate than the strips. For a freshwater community tank you want to strive to keep your ammonia and nitrite levels at zero, and your nitrates below 20 ppm.

It is smart to know the pH reading for your water source as well as your tank itself. While there are ways to naturally or chemically alter the pH in your tank, in some cases you are best off dealing with the hand you are dealt. Most tropical fish can adjust to different pH levels in a well-maintained tank, but you need to do your research to make sure you aren’t setting your new fish up for failure. However, all fish can suffer from swings up or down, so it is important to try to keep a steady pH level.

Taking regular readings of your tank’s water parameters gives you a snapshot of what is happening in your aquarium. You don’t have to guess if something is off; you can see it clearly reflected in the water-quality readings. Then, you have to figure out why and what to do about it.

The next two issues I’ll cover are feeding and tank management. Getting these two things right goes a long way toward keeping your tank healthy and your water parameters in good shape.

6. Overfeeding

Feed your fish once per day and as much as they will eat in a few minutes. Choose a quality flake food, or several and alternate them. Also make sure to include sinking pellets for the scavengers, and algae wafers for plecos, otos, and the like.

Go easy on special foods and only offer them sparingly. I also like to program a fasting day every week, where the fish don’t eat at all, but that’s up to you.

If you keep it simple and worry more about overfeeding than underfeeding you’ll be doing all you need to do to keep your fish healthy and well-nourished. Some fishkeepers like to include veggies, thawed frozen foods like blood worms, or freeze-dried foods. Just like you’ll research every fish you add to your tank, research anything you intend to feed, and learn to do it properly.

Overfeeding is one of the biggest problems in fish tanks, especially smaller ones. To put it simply and crudely, whatever goes into a fish must come out. The more you feed your fish, the more waste they produce. Uneaten food decays, fouling the water. Your bacteria colony can help, and live plants can help, but if things get out of control your fish will soon find themselves in a toxic situation.

Overfeeding can also lead to algae outbreaks. Anything that makes a plant grow makes algae grow, and a nitrogen-rich food supply will turn your tank greener than an angry Bruce Banner.

It can even increase the population of pest snails in your tank, who suddenly have all kinds of extra food in the form of both uneaten fish flakes and algae.

Water changes are an important part of aquarium care that can't be overlooked.

Water changes are an important part of aquarium care that can't be overlooked.

7. Bad Tank Management Practices

Most aquariums really don’t require that much work, once you get them up and running correctly. Thirty minutes to an hour each month is really all the time you’ll spend on maintenance for a large tank, and a small 10-gallon aquarium might require a bit more effort.

However, those few tasks you do need to perform every month are super important. They go a long way toward battling all of the issues discussed above, and if you neglect them your fish will suffer for it.

Your monthly maintenance routine should include:

  • Water Changes: Remember the saying: The solution to pollution is dilution. By removing some of the old water and replacing it with fresh, clean water you are diluting any waste chemicals and helping your friendly bacteria colony. You never want to remove too much water, though. This could cause bacteria to die off and subsequent problems in your tank. I shoot for about 30% monthly in planted aquariums and smaller changes of 20% twice a month in unplanted.
  • Vacuum the Gravel: Uneaten food, gunk, and debris gets trapped in the gravel, and even your industrious little scavenger fish can't handle it all. That’s okay to some extent because remember many of those microbes live in the gravel, but you do need to tidy up from time to time and do a little vacuuming. You can actually do this at the same time you change the water by using a gravel vac. I like the Aqueon Water Changer. I’ve used the big version that hooks up with your faucet for my big tank, but there is also a smaller one for little tanks. There are other brands out there, but Aqueon is pretty affordable and does the job very well.
  • Clean the Algae: Even though you might have algae-eating fish or critters in your tank, they can’t do it all. Every now and then you need to roll up your sleeves to clean the glass and decorations. Excess algae aren’t just an eyesore; they provide food for pest snails and can potentially upset the balance in your aquarium. Don’t use chemicals to get rid of it. There are scrapers and scrubbers designed specifically to use in fish tanks. Some of them float, so they come to the surface if you drop them, and they are magnetic, so you don’t even need to stick your hand in the tank to clean the glass.
  • Filter Maintenance: Don’t forget to clean the gunk out of the filter once a month, but remember that beneficial bacteria live in there, so don’t ditch the inserts unless they are deteriorating or it is recommended by the manufacturer. I typically keep sponges and biological medium until they start to fall apart, and will replace chemical media like activated carbon monthly.
  • Test the Water: Even if things have been going well for a long time it is smart to test the water periodically. It can give you a heads up if things are starting to go south.

8. Disease

I get a lot of questions from people, betta owners, in particular, telling me their fish is doing something weird and asking if it is sick.

Look, fish are weird. They are tiny little creatures with primitive brains and they could do any number of ridiculous, unexpected things. Sometimes betta fish sleep and look like they are dead. Sometimes fish spend a lot of time in one part of the tank. That doesn’t necessarily mean they are sick. If you see your fish acting weird, don’t panic.

But fish do get ill, and dirty water and stress are two big reasons. Sometimes a healthy fish can fight off a disease that would kill a stressed fish. Obviously, this is why it is so important to watch for signs of stress in your fish, as outlined above.

You’ll also want to get familiar with some of the common diseases and afflictions like ich, dropsy, and fin rot. In some cases, as with ich (which is actually a parasitic infection), the disease is contracted from another infected fish. But, in most cases, following the practices outlined in this article such as avoiding overfeeding and stress and keeping the tank and water clean will help a lot with keeping your fish disease-free.

So, when should you start worrying that your fish is sick? If you notice changes to its appearance like ragged fins, bloating, inflammation of the gills or mouth, tiny white spots all over the body, or anything else that clearly isn’t right, get on the web and search those symptoms and see if you can find out what’s going on. Physical indications could be trouble swimming upright (maintaining buoyancy), struggling to escape the pull of the filter, or labored breathing.

9. Mistakes

You’re going to make mistakes. We all do. You can’t beat yourself up over it. Learn, and move on.

I once had a beautiful cichlid tank. My wife and I were going away for a week, so I left instructions with a family member to drop some pellets in a few times while we were gone. Smart! Before we left I cleaned the tank and changed the water. Smart again!

But I forgot to turn the power to the tank back on when I was done with maintenance. This meant my fish spent a week in the dark, in the cold, and with no filtration. Dumb! At least they got fed. Thankfully they all survived, but I really kicked myself for that one.

We all do dumb things. The important thing is to learn from it and not do it again. You may make stocking errors, forget to perform a water change, or accidentally overfeed without realizing it. Or, like me, you may accidentally force your fish to endure a week of technologically deprived hardship.

Go easy on yourself. We are all human, and we all make mistakes.

10. Issues You Can’t Control

Finally, sometimes fish die for reasons that have nothing to do with you. Like all animals, aquarium fish can suffer from congenital issues that doom them from birth. Your vet can check a new puppy or kitten for these types of problems, but with fish, there is simply no way to know.

It is important to review your tank management practices when you lose a fish and make changes where necessary. But also be open to the idea that you may have done nothing wrong, and that your fish was simply destined to live a shortened life.

Because some types of fish are bred in large numbers you will sometimes find issues with the stock in a particular store. If you’ve bought a type of fish from a certain store several times and each has died, and you’ve checked everything listed in this article and you don’t see any problem, it is time to buy from another store. There could be issues with the supplier, or how the store handles the stock. All of that is out of your control.

Don't Give Up!

I hope this article was helpful for those who have lost fish and are trying to figure out why. I tried to cover all of the reasons aquarium fish die, but if you think I missed something let me know in the comments and I will try to help.

If you really want to keep fish, don't give up. Learn from your mistakes, do the necessary research, and get better. I think we all go through that process where we feel like all we are doing is endangering fish, but once you get it right it is so worth it.

Good luck with your fish 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.

Questions & Answers

Question: Are frozen bloodworms good or bad for tropical fish?