There are many reasons why some of your fish have died. You need to review everything that may have occurred in your aquarium
in the last 3 weeks or so.
There are some basic questions that you can ask yourself that will help you to understand what has happened.
1. Check the water quality – are there any unusual results? Are the ammonia and nitrite at a safe level?
2. Check the dead fish and your remaining fish for any signs of infection or disease. Are there any unusual marks, wounds,
spots or growths on them?
3. Have you recently used a water treatment or medicine? If so, how much did you put in? Did you follow all the instructions
4. Have you recently changed the water or carried out maintenance?
5. Have you recently added any new fish or plants?
6. Have you recently changed foods, or added any live foods?
The most common cause of fish health problems is poor water quality, so this might be the initial problem. However, poor
water quality may have caused poor health indirectly, by making the fish more susceptible to a disease. You need to check
for any signs of disease on the fish, because any surviving fish may need to be treated. This may give you the answer
to what killed them. There are a large number of infections, diseases and parasites that can be fatal to fish and it
is very important that you find out what it is so that you can treat the tank.