Why Did My Fish Die So Fast? (Top Beginner Fish Keeping Mistakes to Avoid)

Keeping fish in an aquarium can be fun, and it is one of the most popular hobbies among pet keepers around the world. Sadly, the truth is that average fish only live for 3 weeks after sold from the shops. What can cause the fish die so quickly?

In at least over 90% cases, fish die early because of the following common beginners’ mistakes:
1. Tap water was not treated with aquarium water conditioner.
2. Shock from sudden change in water temperature, and/or water chemistry (or no heater at all for tropical fish).
3. Ammonia and/or nitrite poisoning. (or no aquarium filter at all)

For Reason #1 – Fish Died to Untreated Tap Water

You must know that fish can’t survive in untreated tap water. Tap water contains chlorine and chloramines. While Chlorine is said to be able to evaporate slowly if you let the tap water sit for a few days, chloramines is a different story and it is there to stay. Both chlorine and chloramine can and will kill the fish.

The solution to this problem is very simple. Buy a bottle of aquarium water conditioner. Years ago when I was a kid, people used solid crystal like water conditioner. Nowadays, more water conditioners are in liquid form.

I have personally used Seachem Prime, Kordon NovAqua+, Tetra AquaSafe, and Hikari Ultimate.

While they are all good and work just fine, I recommend Prime for new tanks because it can not only do the typical tasks of water conditioner which is to neutralize chlorine and chloramine, but it can also detoxify ammonia and nitrite for up to 48 hours, which can come in handy for a new tank.

For Reason #2 – Fish Died to Shock From Sudden Changes

Most fish can adapt to a certain range of water chemistry (PH, hardness, etc.) and water temperature without problem if given time. However, many new fish keepers do not know this. They will simply “dump” newly bought fish into their fish tanks immediately. The water in the tank obviously has different chemistry makeup and temperature compared with the water in the plastic bag that came with the fish. Unlike human being, fish are cold blood animal, and they can’t regulate their own body temperature like we do. Sudden change of water temperature can “shock” the fish and even cause them to die! The water PH and hardness are also extremely important; the sudden drastic change in either of them can result in the fish’s death.

Solution: Before you open the plastic bag, put the bag into the fish tank water. Let it sit there for at least 20 minutes for the temperature of the water in the plastic bag to become the same as the water in the tank. Many novice fish keepers do that after listening to the sellers at the fish stores, however, what they miss is the next step.

Now the water temperature is the same after the fish bag has been sitting in the fish tank water for over 20 minutes, but the water chemistry such as PH and hardness are still different! What now?

This is what I usually do. Use a plastic fish container (the little plastic box the fish store salesmen use to keep the fish caught from the tanks before putting them into a plastic bag), and dump the fish & water from the plastic bag into the container. There is no need to transfer all the water if there is too much water for the container to handle. Then get rid of some water from the container (throw that water away, do not put it into the fish tank), and refill the container with water from the tank.

Wait for 10~15 minutes, then get rid of some more water from the container, and refill it with the water from the fish tank. Repeat that process every 10~15 minutes with no more than 25~30% of water removed/added each time…until the water in the container is mostly from the fish tank instead of the original water from the plastic bag. In this way, the fish have the time to slowly get used to the new water chemistry. The whole acclimating process usually takes me about 1.5 hours. For more sensitive fish, it can take 2~3 hours before I feel safe to add the fish into the tank.

The above scenario is based on the assumption that you have a heater if the fish you keep are tropical. It does not matter how slowly you try to make the fish to adapt to the new temperature, tropical fish (such as betta) will die if there is no heater to keep the water at the needed tropical water temperature which is usually around mid 72~80F+.
If you are looking for a heater, I would recommend Rena SmartHeater. I use a 50w heater in a 40-gallon tank, and the temperature is very stable since I have good water flow around it.

For Reason #3 – Fish Died to Ammonia and Nitrite Poisoning

Ammonia and nitrite poisoning is a more complex problem. First, you MUST have a filter running 24/7 if you plan to have fish. Without a filter, no fish will be alive for long. That is certain and there is no way to get around it. If you are looking for advice on a filter, I would recommend AquaClear power filter for small sized tanks of up to 30 gallons. They are very reliable.

For medium to large sized tanks, I recommend canister filters. They are more efficient at biological filtration, and they make no noise at all, but they cost more.

I personally use an EHEIM Classic 2213 for my 40-gallon tank, and I am very pleased with it. It is a high quality German brand.

Now let us talk about why we need a filter and why there is ammonia and nitrite poisoning. Fish produce ammonia as a natural waste. Ammonia is toxic and it will harm the fish.

In a natural environment such as a lake or a river, there is so much water that ammonia gets removed quickly, but in a closed system such as in a fish tank, ammonia builds up and become more and more concentrated quickly! It burns the fish’s gills, and it can kill the fish when the concentration is too high or exposed to low concentration for too long. Usually in two to three weeks, the level of ammonia concentration in a fish tank will be sufficient to kill the fish.

Now what? How do we remove the ammonia from an aquarium?

The answer is you do not need to physically remove it. In a well established aquarium, there are enough naturally occurring bacteria that feed on ammonia as food. Most of these bacteria live on the filter media where there are lots of surface areas. (These bacteria only colonize in the surface areas)! The ammonia feeding bacteria will convert ammonia to nitrite. However, nitrite is even more toxic than ammonia!

Now what? There is a different type of bacteria feeding on nitrite, and convert it into a much less harmful form – nitrate. The second type of bacteria also colonizes the surface areas in the fish tank, and just like the first type of bacteria, they mostly stay on the filter media because that is where the most surface areas are in a fish tank.

Nitrate is completely harmless to the fish until there is extremely high concentration, while ammonia and nitrite must be kept at 0 at all times, if you want your fish to live.

However, it takes up to 6~8 weeks for these two types of bacteria to grow into sufficient number in order to keep the ammonia and nitrite at 0, if…and only if there is a constant source of ammonia. By having fish in the aquarium, there is a constant source of ammonia, but the fish will usually die before the 6~8 weeks is up!

What did I say? You will never have remotely close to enough bacteria to feed on ammonia and nitrite if there is no fish (source of ammonia). But your fish will also die if you add it into the fish tank before there is enough bacteria. It makes no sense, does it? It surely sounds like nobody can keep fish alive at all if the fish tank is newly set up.

It actually makes perfect sense.

Solution to the #3 problem? No fish should be added into a new aquarium until you do a fishless nitrogen cycle!

To test your water chemistry for ammonia, nitrite, nitrate, and PH, you need a liquid water test kit. (Paper strip kits are not accurate at all.)
I use API freshwater master kit.

For saltwater aquariums, you need a different test kit called API Saltwater master kit.

For detailed information on nitrogen cycle, please read Fishless Nitrogen Cycling.

Lets hope more novice fish keepers do things right, so there will be fewer fish die in home aquariums.

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49 thoughts on “Why Did My Fish Die So Fast? (Top Beginner Fish Keeping Mistakes to Avoid)

    • Over a year is definitely not enough for most fish species. Most fish can live for much longer than just a year. Depend on the actual fish species, the typical aquarium fish’s life span is anywhere from 2~10 years. Some of them can live much longer. If only sometimes your fish live over a year, then there is definitely room for improvement.

  1. Ive had my fish for ten years got a bigger tank put some of their old water and new water.i have done this many times. Today i did the same and they all died. They were so big n beautiful im just heart broken!

    • Maybe there is too much new water? If the water hardness and PH had changed too much, the fish would not be able to adapt and it might cause them to die.
      Whenever you do water change or tank transfer, it is the best to keep at least 50% of old water. The fish should be given time to adapt…instead of just dump them in.

    • You have to be more specific about your situation. Please come to our forum to give more detailed information such as your tank size, filter type, how long have you had your fish, how often and how much do you feed them etc. Without any information we can’t answer your question.

        • If you looked at the previous posts, he/she/or maybe you have asked why did your fish die. Without further information on what you have, and what you have done so far, there is no way to pinpoint the specific cause. I did ask for some more information in details in order to answer the question. The “further information” he/she/you replied was “I got girl and boy fishes”. That was indeed not helping to fill me in on the situation.

  2. I have had a well established 100 litre tank for nearly a year. It’s beautiful. Until today. Last week well 6 days to be accurate I Brought a nee fish to join my panda moor and black moor.I choose a pearl fin gold fish *the puffy looking ones as advised by the pet store. However today he was bobbing about the top at first I suspected swim bladder but within an hour he was dead. Tested waster with a liquid testing kit and levels were OK. Did a 40% water change after removing him and retested a little while later still all OK. So why did he die?

    • Perhaps the new fish was not able to adapt to the new water in your aquarium. The introduction of new fish is one of the hardest part. If the fish can adapt, they live. If not, they often get sick or die.

  3. I had a shrimp in my fish tank for over a year. I changed the water in the tank several times. The shrimp developed bumps on the back and died. Any ideas why it died?

    • Sorry, there can be many reasons for the shrimp to die. I am unsure of the actual cause without a lot of details such as fish tank size, the filter you use, heater you use, the food, and the actual species of your shrimp. How often do you change the water, what water conditioner do you use, etc.

  4. I brought new fish tank stones and 6 fishes starting 2 day my fishes are good i give them 1 time food in a day but my 2 fish die early morning after that i think reason will be food is not sufficient for theme then i give them food 3 times in a day after five day 1 more fish at evening day im change water every 4-5 days

    • Hi,
      You only need to feed your fish once a day. Feed them no more than what they can finish (all of it) within a minute. Overfeeding is one of the most common problems in fish keeping and contribute to a lot of fish deaths.

  5. Only now do I know what were all the reasons behind my fish sudden death everything I bring new ones. Thanks to the admin for the detailed analysis of the problem. Got a question here. How should we feed our fish. Can there be any variety or it should only be the same packaged food we were given during our purchase.?

    • Hi Naveen,
      Thank you for reading the article.
      It is the best to get some high quality fish food. We also have articles about fish food on our blog. You may check them out to find your answer. If you still have questions, feel free to ask them on our forum. 😉

  6. I have had my guppy and platy fish in my pond(which also has a water Lilly) for nearly 11years and they multiplied really fast. But when I put new goldfishes in it (25 or sthg, my cousin bought me), both the goldfish(in gps of 7) and guppy and platy are dying after 3/4days… Pls tell me why… can’t bear to lose more…😞

    • Hi Jane,
      How big is your pond? Do you have a filter system running 24/7?
      Goldfish are known for their messy eating/pooping habits and the large amount of ammonia they can produce. It is very likely the newly added goldfish have overstocked the pond. It is causing an ammonia spike, which will eventually kill all fish if it is not fixed.

  7. It’s an average pond and no I don’t have a filter system… now there’s only some 5/6 gold fish in it (rest died)… will that help?? Or should I remove the goldfish??

    • Without a filter system, ammonia will build up and eventually kill all fish.
      You first need to figure it out how big your pond is, then get a filter system for it accordingly. Please check out our article on Aquarium Filters to see why you need it. No fish can be kept alive without a filter. Unless your pond is huge with tons of aquatic plants while having only a few fish.

  8. Hi admin,
    I had 4 Gold fish 2 tiger fish and 2 big shark fish they were about 2 weeks then the started dying any reason why they died and after my Gold fish started dying my shark fish were eating them.

    • Too small of fish tank perhaps? What filter system are you using?
      “Tiger fish” can be many different fish species. I need the exactly fish species to be sure. Same goes for “big shark fish”. They might not even belong in the same fish tank since their natural habitat are different.

  9. I have a new tank . Set it up 4 about wk b 4 buying my fish. I have a tank heater , checked everything , the PH , alkinity, chlorine, ammonia & all b 4 I bought my fish. I bought 2 tiger barbs , 2 clown loaches , 2 angles , 2 sharks and 1 sucker. Brought them home let the bag sit in the water 4 -2 hrs then put them in the Tank. 3days later my sucker fish was dead.
    The next week we went and got another sucker fish and 3 dalmatian mollies. Did the same with the bags. Did not add their water to my tank tho. We noticed that one of my clown loaches had ick . We treated the tank , both my clown loaches died that day . Then the next day 1 of my shark fish was dead and my sucker that I had for 2 days was dead . I cheked all my levels and every thing was spot on . Would you know why they all died at once. I had already put my filter back in and took out 25% of water and treated it b 4 adding new water to tank after ick treatment. Plz help me . I love fish and fish Tanks. This is my first attempt at owning one and I really don’t want to give up. Thank you for all the help you could give me .

    • Maybe the treatment you used was too harsh on the fish? Treating ick can be done without using medications. Check out our article on ick.

  10. We have all kinds of fish. But we get new ones, and we float them, then the water thing, guess what the new ones die. The water is conditioned. The others dont die and they was new once upon a time. So what now!

    • Did you acclimate the new fish well? They need more than just the same water temperature. Other factors such as water PH, hardness, and even nitrate concentration can affect fish if it changes all the sudden. You need to add fish tank water into the new fish container little by little until it is almost all fish tank water in the container. Fish need time to adapt.

    • Hi,
      Many things could have gone wrong.

      We need more information to pin point it. Such as the actual tank size. The fish species. What water conditioner did you use. What filter do you have.

  11. i have a flat fish tank outdoor my house with a capacity of 1000 litres. i have put around 120 pet fishes (the gold, gold with black shades, black and white pet sharks). Everyday one or the other fish die wen i see in the morning. I add additional water to the tank if the level is below the normal. Can u plz help to my fish sustain in the tank.

    • Do you have a filter system in your fish tank? If so, what filter is it?
      Are you using an aquarium water conditioner? If so, what is it?
      Have you added water conditioner to the new water and let it sit to have the same temperature before adding it to the fish tank?

  12. I had two black moore fish in my tank which were perfectly fine until a couple of day back.I did my usual (50%) water change and my black moore suddenly started to fli-over and both of them died.
    I have been using the same water source. This is a fairly new tank.Approximately around a couple of months old.

    Tank Capacity: 40 Liters.

    2 – Fancy Gold
    2- White Gold
    4- Bala Shark
    2- Small Koi Carp

    • Maybe you did not acclimate them well during the water change? You can easily shock and kill your fish if the temperature, PH, hardness, etc. of the new water is too different from the old water from the tank.

  13. I had 16 fishes in my tank and now I have just 10 remaining. The fishes are having a white border on their gills and fins I don’t have a filter system ..i donno how it happened overnight..can you explain the reason?!

    • How big is your fish tank? It is likely you are overstocking your fish tank.
      Then you have mentioned “no filter system”. That is a big no in fish keeping. No aquarium filter = no way to keep fish alive for long.

  14. I have a 29 gallon fish tank went to pet smart had my water tested they said it was great I even let it sit for 1 week like they told me to bought 10 tropical fish and 1 alg eater did what they told me to.After 1 day one died after 3 days another one after 1 week one more and last night my alg eater was dead I got 5 more left.what can I do for the rest to survive.

    • These chain pet stores never give the right advice. Their employees are simply not trained to answer professional questions about pet care.
      If you read the article of “Aquarium Nitrogen Cycle and fishless Cycling”, you should know the reasons.

      Let the fish tank “sit” will do nothing.

  15. Hi!
    I have recently bought a fish tank
    My fish all have died in a week or two until recently I bought new fishes and they seem to be doing pretty well! Your article helped a lot. Its been 3 weeks All still happy and active. I just need tips to make sure they survive Should I do very partial water changes or complete water changes? I also have guppies in a fish bowl Tips for them? My fish tank is 30 gallons. Anyway thanks after all

    • Hi Faris Ahmad,
      Never do 100% water change. Fish will get into shock and die easily if you do that. They do not like large swings in their environment. Stick with 30~50% partial water change weekly. Make sure your filter system and heater are running properly.

    • Hi chanula,
      There is not enough information for me to find out the cause for your loss.
      You need to tell me more details first. 🙂

  16. Okay, I realise I may be doing the new hobbyist obsessive thing (which has brought me to this thread!) but I’ve got my first aquarium fish today (cardinal tetras) and for like 15 seconds or something I failed to realise that one was left in the bag after I released them. Now, I won’t ever release them the same way again (I’ll lower the bag so they have to swim up and out) so that’s a lesson learned, and I did the water temp/ph thing but I’m so worried that I might have stressed or harmed this little fish. He’s swimming along with his buddies now and looks perfectly okay but I feel terrible. 🙁

    • Hi Elle,
      If the fish is fine, then there is no need to worry about it. The fish will act weird almost immediately if they are in shock. Otherwise, they are most likely fine.

  17. i have 5 fishes and one of them died this morning i dont know how but she was not eating anything and whenever i gave her food other fish ate all food. can you tell me how to give them food and what must be the reason behind her death

    • hi abhishek,
      How long have you had your fish? Size of the fish tank? What filter system and heater? What exactly fish species are they?
      I need more information to try to find out what’s wrong.

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