Why do my fish keep dying?

There’s nothing more frustrating for a fish owner than looking into your aquarium to see one of your fish floating lifelessly at the surface.

It can be even more frustrating if over the next few days more fish start to drop dead, leaving you wondering what is making them die and whether you have done something wrong.

If you have lost a few fish in a short space of time it could of course be a coincidence. Many species of fish only live for a year or two on average, meaning that death is inevitable at some point.

But it is worth keeping an eye on the situation in case there is something in the aquarium that is causing the fish to die.

Why your fish might be dying and how to keep them alive.

It could be a case of New Tank Syndrome. This is the term that is used when fish become ill or die in a newly established aquarium. It is natural for fish to produce ammonia and this is created when waste is released into the water by the fish.

Excess fish food and debris also rot in the tank and this too can increase ammonia levels.

Some people choose to do a fishless cycle when they first buy a tank in order to establish the aquarium and prepare it so that it is a safe environment for fish to thrive in once they are introduced.

A fishless cycle can be a complicated process, but it basically involves mimicking the changes that would occur when fish are added to a tank – but without the fish.

This means that you need to add ammonia to the tank yourself in order to mature the filter so that when you do add the fish, the environment won’t cause them any harm.

However, the majority of people add their first fish to an aquarium a week after setting it up, without any problems. Problems can arise though if your aquarium is not properly cared for. If you’ve recently lost fish or your fish have become ill, here’s our guide to potential problem causers.

Poor water quality

Water quality plays a huge part in the success of your tank, and if it is not up to scratch, your fish can pay the ultimate price. Ammonia and nitrite are just two of many common problems that you might come across while caring for an aquarium and both can harm your fish.

A number of things can lead to ammonia, such as fish waste, leftover food rotting at the bottom of the tank, and dead fish not being removed promptly. Overfeeding your fish is a habit that is definitely worth avoiding.

Insufficient water changes

If you don’t conduct regular water changes, then fish waste and leftover food will remain in the tank. A fish tank filter plays a large part in removing food and waste, but it can still help to regularly add fresh water so that problems don’t continue to circulate.

Water changes need to be done at least once a week, and you should get into the habit shortly after purchasing your fish tank, rather than waiting for problems to arise before you start to change your aquarium water. To make sure that your water quality is good enough for your fish, using a test kit regularly can really help.


It may surprise you, but just like humans, fish can become stressed easily and this can be bad for their health.  The more stress that they experience, the more prone they are to catching diseases and the more they struggle to fight these illnesses off.

Stress can be caused by disturbances such as loud noises, unnecessary substrate changes, hands being placed in the fish tank and bullying from other fish. It’s best to avoid disturbing the fish tank unless absolutely necessary.

To prevent the fish from picking on each other, it’s worth checking that the fish you buy are compatible before placing them together in the tank. Some species of fish can’t live in harmony and you might find one or two fish chasing others around the tank or nipping their fins.


Overcrowding is a common fish killer. Many aquarium owners can get carried away when purchasing fish, buying far too many for the size of tank they have. This not only gives the fish less space to move around in, but it also increases the chance of there being an ammonia problem.

A good rule to follow is just one cm of fish per litre of water. This helps you to work out how many fish you can have while taking on board the fact that not all fish are the same size. Even if you don’t have too many fish in your tank, adding several fish at once can cause ammonia levels to spike and such a sudden and dramatic change can kill the fish off quickly when they run out of clean water to swim in.

It’s best to gradually introduce your fish to the aquarium a few at a time, making sure that you acclimatise them properly. Take a look at our guide explaining how to add fish to your fish tank for more information on acclimatisation.


You may worry that your fish are hungry and will die if they aren’t fed enough, but it is actually more harmful to feed your fish too much. Overfeeding is a common problem and one that can be dangerous even if you’re only guilty of it on occasions.

Uneaten food will sink to the bottom of the fish tank and will gradually decompose, increasing the amount of ammonia in the aquarium. One way of sucking up all of the uneaten food is by including a bottom-feeding fish like a catfish to scoff any leftovers.

Regular water changes can also help to minimise any effects of overfeeding. But prevention is better than cure so it is best to not overfeed in the first place.

Insufficient chemical filtration

Remember to use dechlorinator and stress coat when doing a water change to make sure that the water is safe for the fish. Adding water straight from the tap without treating it first is a common killer of fish

Lack of warmth

If you have tropical fish, you need to maintain a certain temperature to ensure that they don’t get too cold. Fish tank heaters do the job perfectly and the majority will regulate themselves to make sure that the water doesn’t get too hot or cold.

Contaminated water

Water can become contaminated quite easily, and the slightest trace of foreign material can cause problems for the tank inhabitants. One of the most common ways that a fish tank can become contaminated is when doing a water change.

We recommend using the same bucket for water changes and for water changes only. Never wash this bucket using cleaning products as the slightest drop of washing up liquid can contaminate the water, for example. Water can also become contaminated if you add the water straight from the tap without using dechlorinator to remove chlorine and other toxins.


When an illness begins to take hold of your fish tank, it can be difficult to control, and before you know it you can lose several of your fish within the space of a few days and in really severe cases, hours.

Thankfully, almost all fish diseases, parasites and bacteria can be cured with treatments and medicines. Make sure that you don’t overmedicate though as this can cause more harm than good.

Disease treatments can be a valuable addition to your aquarium kit when your fish start to get sick or if they die. These treatments can target the problems and prevent them from spreading.

But first, you must of course work out exactly what illness it is that your fish are suffering from.

What are the most common fish diseases?


Formaldehyde is a great disease treatment to conquer parasites and flukes. It can be teamed with NT Labs Malachite to combat parasite infections such as Ichthyopthirus white spot, Trichodina, Costia and Epistylis.

Anchor worms 


  • Scratching against objects
  • White/green threads emerging from scales
  • Inflammation of the scales where threads appear

Disease treatment

An Anti-Worm treatment can help to protect your fish from parasitic worms or lice. Anchor worms, skin worms, gill worms and tapeworms can all be eradicated with the correct dosage at regular intervals. It’s important to follow the instructions included with the disease treatment so that you don’t overmedicate the fish.

White spot (also known as Ich or Ick) is one of the most common parasitic diseases that tropical fish can face. The majority of fish enthusiasts will have to deal with it at one time or another, but the problem can be solved with appropriate disease treatments if caught early enough.


  • As you’d expect, white spots can become visible on the fish’s body.
  • The fish might also rub itself against plants and decor as if it was trying to reach an itch.

Disease treatment

API White Spot Cure can effectively cure white spot in tropical and freshwater fish. It also provides a barrier against secondary infection by providing a slime coating on the fish.


  • Frayed and damaged fins that look as though they’re been torn.
  • Red markings where the fins are damaged.

Fin rot isn’t exactly a disease in itself, it is usually a symptom of a different illness. It can be difficult to determine the cause of fin rot but it can include fighting with other fish, stress, bacteria, parasites, ammonia poisoning and poor water quality.

Disease treatment

An Aquarium Anti Fungus and Finrot disease treatment can quickly treat fin rot, mouth rot and cotton wool disease in fish. As soon as you notice that any of the above conditions have affected your tank it is important to act quickly in order to tackle it, prevent the spread of the disease and decrease the likelihood that the fish will die.

What else can I do to prevent the disease from spreading?

Fish quarantine

As well as using an appropriate disease treatment, quarantine can be used to prevent the spread of diseases from one fish to another fish in the tank. A quarantine tank can be an effective way of preventing other fish from getting ill and helping treat the diseased fish.

The easiest way to prevent your fish from getting ill is by regularly taking a bit of time to maintain your aquarium and keep it clean. Weekly water changes and test kits can go a long way, as can avoiding overfeeding.


  • joe Posted 29/04/2014 at 4:00 am | Permalink | Reply

    I thought the picture of the toilet was your way of saying another way to prevent spreading of disease was to flush them down the toilet haha.

  • Dennis carter Posted 09/05/2016 at 2:56 pm | Permalink | Reply

    Hello I have white spot in my fish tank I’m using anti white spot & fungus but my fish keep dying do you know the reason why

  • Veiji Posted 21/11/2017 at 5:49 pm | Permalink | Reply

    I got all of my fish at the same time. I had two dalmation mollies, one black, one white, a gourami, a black moor and a guppy. One dalmation was pregnant before she died, when she was found, no babies were in sight and she was a lot smaller. Second to die was the white molly, it was found floating in the plants, it never moved around and was just sluggish, today was the black molly, the guppy and that one hung out often, i also found that one in a plant, dead. I treated the water, have a heater and filter, but they are dying off slowly and i have only had them for 11 days. I dont know what Im doing wrong.

  • Dhwanee Kolage Posted 29/03/2018 at 2:38 pm | Permalink | Reply

    i recently bought 5 fishes which included 2 guppy baby fishes, 2 Angel fishes and 1 swordtail fish. Just the next day i found both the guppy fishes dead followed by the swordtail fish. i have no idea what is going wrong. PLEASE help.

  • Monalisa Montalvan Posted 01/04/2018 at 6:28 pm | Permalink | Reply

    I gave our Pastor a present. I gave him Aquarium included 6 pieces of gold fish but suddenly they died every other day. 2 remaining gold fish left. Is it okay to put aquarium in a Church? Is it stressful?

  • Sydney Posted 02/04/2018 at 2:28 pm | Permalink | Reply

    Yes they my be dying because one had it and you did not put the treatment in fast enough so it could have already spread, I’m sure you have found an answer to this question it is 2 years old btw: if all your fish die it may not be a bad idea to fully clean out the tank
    Hope I helped 🙂

  • Sprite Posted 03/04/2018 at 6:33 am | Permalink | Reply

    Goldfish produce LARGE amounts of ammonia, you should get a water treatment or ammonia reducing pebbles (you put them in a filter bag, and then in your filter) from your pet store that reduces ammonia. Also you can buy 5in1 test strips to test other water qualities: nitrate&nitrate, alkalinity, hardness and ph

  • Sara Posted 08/04/2018 at 1:33 am | Permalink | Reply

    I got three guppy’s an one of them died on the second day. It was laying on the bottom of the tank and died ‘resting’ on a rock. The other two were fine. Now after one guppy died another is doing the same. I’m afraid it might die as well. What is happening? And what I am doing wrong? (The tank has s filter thou and before I put them in I put this thing that makes tap water safe and aquarium salt)

  • Nsfworanyone Posted 01/05/2018 at 2:52 am | Permalink | Reply

    A couple months ago my family got this huge 50 gallon tank with, three micky mouses and 2 stripped fish, and then we got another batch of fish and it all turned diwn from there they new fish got ick and some died but we managed to save the rest, we’ve gotten two more batches, the third included a sucker fush and some others i can’t remember and then in the new batch we got an albino sucker fish(which has also apparantly died) as well as an oranda goldfish, for some reason alot of our fish keep dying and this past week one of our micky mouses was breathing hard and would not swim, it would just float down to the bottom of the tank and unfortunately it died today(though it was 4 years old i read they only live 5 years) , me and my family have no idea whats going on, we have good filters plenty of space, and 50 gallons for relatively small fish who would all fit in my hand, at least 10 or more, we only feed them once a day (after algea staryed to grow on the tank’s sides) so i don’t think we’re over feeding them, we add fresh water and clean it once a month, it doesn’t look like they have the diseases mention or have ick, we don’t have young children in the house so no one is sticking their hand in unless absolutely neccesary, though maybe our dogs are stressing them out, sorry for this long paragraph, but we really love these fish and its upsetting to think that maybe we’re doing something wrong thats killing our fish

  • Michelle Posted 08/06/2018 at 12:31 am | Permalink | Reply

    Hi I installed my uvc steriliser the wrong way and my fish are now seeming to die. they have turned black and are looking paralised with mould on their skin. What can I do?

  • Tisha Leonard Posted 27/06/2018 at 3:24 am | Permalink | Reply

    Just set up my 20 gallon tank for my kids. The lady at pets mart said wait 1 week before introducing fish into the tank. I didn’t listen. My kids wanted a fish so we thought a goldfish ….well they lasted 3 hours in the tank and they were dead. Had the tank water tested my pets mart and they said it was fine for goldfish. I bought 2 more ….they are almost dead. I don’t know what I am doing wrong. Temp is at 76 or so. New filter new everything. Should I wait the whole week and try again?

  • Matthew Posted 01/08/2018 at 9:07 pm | Permalink | Reply

    I have lost around 20 fish in the past 3 days. I have a 230l take and I don’t know why. I’ve had the water tested and all the levels are fine. I have no idea what’s going on. I’ve done water changes and that’s not sorted it. (Yea I used tap safe etc). Could it be the food that I give them (frozen bloodworm). Any help or advice is much appreciated.

  • Lorin Posted 11/10/2018 at 5:56 pm | Permalink | Reply

    My tank is fairly new. However, I had put the filter media in another tank for 2 weeks before setting it up. So that should have provided some bacteria for the tank. The tank has been up for about a month and in the past week I have lost 5 fish. I have a master test kit and I have tested everything and it says the water quality is good. The PH is a little high but not to the point it would be causing this issued. I have no idea what is killing everything off.

  • Brandon Posted 20/10/2018 at 9:31 pm | Permalink | Reply

    I’ve lost around 5 Bettas to dropsey, my newest female I’ve only had for a week already has it! … last night t I checked her water perameters and it’s all fine,
    I don’t know what I’m doing wrong.

  • Arjun Posted 14/12/2019 at 1:54 pm | Permalink | Reply

    I bought I pair of white mollies but the female fish died within 6 days . And I bought another female but it had also died the very next day

    • Jeremy Gay Posted 14/01/2020 at 1:29 pm | Permalink

      Mollies are best when females outnumber males by at least two to one. The male may have harassed the female to death, then the following one. Mollies need a high pH and salt in the water. The pH may have been too low for them or the females were already sick when you bought them.

  • Annalien Posted 15/12/2019 at 9:15 am | Permalink | Reply

    I have a 180L tank. My water test 100%. And my fiche die one by one. Start to spin around and then it is gone. What can be wrong. I lost 18 fish in about 3 weeks.

    • Jeremy Gay Posted 14/01/2020 at 1:27 pm | Permalink

      Spinning fish is usually down to Nitrite poisoning. To lose so many fish in such a short space of time I would still suspect water quality and New Tank Syndrome, but failing that, a parasitic infection or poisoning of the tank water from an external source.

  • Morgann McGee Posted 21/12/2019 at 4:58 pm | Permalink | Reply

    I had 6 neon tetras, 4 guppies and a betta in my tank. i have had the betta for around a year and the tetras nearer 8 months and the guppies are new. when i bought the guppies, i moved all my fish into a new tank, the next day 1 guppy had died, a week later another guppy died. i checked the water quality and did regular water changes which was fine but then my betta died. i had enough and was really panicing so i moved the remaining fish into their old tank. they all seem to be doing ok so far, but i dont know what has caused my other fish to die. please help.

    • Jeremy Gay Posted 14/01/2020 at 1:24 pm | Permalink

      If ammonia and nitrite were anything above zero then it could be enough to stress and kill the fish. If water quality was perfect in the new tank then the Betta may have killed the guppies, or the guppies may have been carrying a disease. The Betta may have died from old age (they are short lived,) or from contracting disease from the guppies.

  • Ashley Graham Posted 22/01/2020 at 1:07 pm | Permalink | Reply

    I was given a 29 gallon top fin aquarium for Christmas and I set the tank up took my water got it tested and bought some glow tetras 3 then died with in 4 hours. Water tested fine temp was 78°. We got 3 more fish same kind and they lasted about 5.5 hrs. I switched all my water to natural spring water had my PH was reading high so I took care of that got 3 more glow tetras and they lasted about 7hrs. I retested water and decided to just let the take keep running empty of fish changed filter too. I’ve add all the things I need to add to the tank and let it run even longer and just got 3 more tetras and they seemed to be doing better lasted 8-9hrs but are dead again….

    Any ideas folks any ideas….

    • Jeremy Gay Posted 28/01/2020 at 9:59 am | Permalink

      For newly added fish to all die in such a short space of time I would look to source water or poisoning. Try emptying and refilling the tank with reverse osmosis water and a remineralising agent. Add some beneficial bacteria on the day you add the fish and test temperature, pH, ammonia, nitrite and nitrate daily for the first few weeks. If you still lose fish within hours of adding them then the cause could be a poison of sort sort either on the tank decor, or the tank itself, or in the room. Avoid insecticides, air fresheners and any foreign (non aquarium safe,) objects which could be leaching into the water.

  • Molls Posted 05/03/2020 at 2:26 am | Permalink | Reply

    I have a 45 L tank and left it running for almost two weeks before buying fish. The first fish I got were 6 neons and they have all survived for about the month I’ve had the tank. Then I got a Bristlenose and 6 Mollies. Within 2 days I had baby Mollies and separated these for the main tank. Then I brought 3 Ram Cichlids which look quite settled and they getting their bright colours! Then all of a sudden one of the Mollies died so I did a full clean of the tank but now all 3 Rams died in the same night and my Bristlenose died today!! The Rams the evening before they died didn’t seem to want to eat and sticked at the top of the tank then the Bristlenose did the same thing the evening before he died! I don’t understand I use all the correct additives when cleaning out?! Please can someone help!!

    • Jeremy Gay Posted 10/03/2020 at 8:52 am | Permalink

      It sounds like a water quality issue to me. Rams are sensitive but Bristlenose catfish are very hardy. To lose a Molly, the Rams and a Bristlenose sounds like you had an ammonia or nitrite spike. What you haven’t mentioned are the results of water testing. A new aquarium can take 6-8 weeks to cycle and fully mature, and within that time it should be stocked slowly with a few small, hardy fish, and the water tested regularly, (and logged.)16 fish is quite a lot of fish to add to a 45 litre aquarium in a short space of time. Conduct a 50% water change, add some beneficial bacteria and don’t clean the biological filter, to allow it to build up a biofilm. Stick with the fish that are left and only consider adding more after a week or two of test results showing zero ammonia and nitrite. Bristlenose will outgrow a 45cm aquarium and really need one of 80cm or more. Rams need mature aquaria with a low pH and warmer than usual water.

  • Wendy G Weaver Posted 23/03/2020 at 12:46 pm | Permalink | Reply

    I bought two glofish yesterday for y kids we fixed the new tank the day before ,took them home they lasted til last night, mysons green one went first then I woke up to my daughter’s red one dead. I put conditioner in water in bowl and then added it to tank ,before I got fish took a sample to PetSmart the pH was good nitrates were not at right level but she said it was because it’s a new tank. What could be the reason they passed ? I want to go get the kids more today I just don’t want t them to die too, if u know what is wrong cN u o!ease tell me these are my first fish since I was a teenager pleased help I have no idea what I’m doing

    • Jeremy Gay Posted 31/03/2020 at 9:49 am | Permalink

      It sounds like New Tank Syndrome to me. A new tank should be set up, filled with water, dechlorinated, brought to temperature (and checked with a thermometer,) then have beneficial bacteria added on the day that the first fish are added. If your fish died and there were “nitrates” in the water, then it was probably toxic nitrite, meaning that there was not enough beneficial bacteria in the water to convert the fish waste. Test for pH, ammonia, nitrite, nitrate and temperature before trying again. Ammonia and nitrite should be zero. Nitrate less than 40ppm, pH 7-8 and temperature 78f. Make sure you acclimatise new fish properly too by floating the unopened bags on the surface of the water for twenty minutes, then introducing tank water to the nag water over another twenty minutes. And if you do all of the above and you still lose fish, they could be sick when you buy them. Start with some hardy fish like standard Zebra danios, Danio rerio.

  • Marina Posted 14/04/2020 at 12:27 pm | Permalink | Reply

    So I’ve had these fish for at least two months and one of my fish just died, he was laying at the bottom of the tank while another fish was beating him up. Before that he was laying there not look too well? Was he sick? Was it the water? The next day the white one died. She just floated to the bottom. Now my two left remaining fish aren’t really eating. Are they stressed? Is it my water? Please help.

    • Jeremy Gay Posted 21/04/2020 at 9:07 am | Permalink

      Wow! It could be a whole host of things. What fish are they? What size tank is it? How long has the tank been set up for and how is it filtered? Obtain test kits for pH, Ammonia, Nitrite and Nitrate, gather some more information about the fish species and get back to me.

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