The key to managing disease in your fish tank is to observe your fish. Watch your fish every day but at the same time look for signs of ill health and disease.

Are they swimming where they should be in the tank for their type of fish, or are they sat motionless on the bottom or hanging just below the surface when they are usually active in midwater? Are their fins erect and free of white fluff and erosion? And are their bodies free of mucous, white spots or red ulcers? If they’re showing any of the above symptoms your fish may have a disease. 

Test the aquarium water first

The first thing to do if your fish don’t look right is to test the water for pH, ammonia, nitrite and nitrate. Poor water quality is the biggest killer of aquarium fish so if your test kit reveals anything above 0.0ppm for either ammonia or nitrite, that’s the most likely cause of their ill-health. Remedy water quality first and they may get better, but they may also develop a secondary infection from the stress caused by living in poor water. 

How to diagnose fish diseases

If the water is good you need to know what disease it is that the fish have got so that you can treat it with the right medicine. Unlike when your cat or dog is sick, ill fish aren’t taken to the vets and prescribed veterinary medicine. Instead take a picture or video and ask at your LFS or online in a tropical fish health group. Some diseases are pretty easy to diagnose like Whitespot or Fungus, but many parasites are invisible to the naked eye, making them difficult to diagnose on looks or symptoms alone, and they’re the ones that are often treated too late. 

How to treat fish disease

Once the problem has been diagnosed, treat the whole aquarium with the right medication. It's critical to know the volume of the aquarium so multiply the length by the width by the height in cm then divide by 1000 to get the tank’s maximum volume in litres. Leave filters and airstones running (extra aeration helps sick fish,) but remove carbon as that soaks up medications, and turn off UV sterilizers, which break medicines down. Just like a course of medicine from the doctor’s, use as directed and for the recommended duration, as some parasites may take seven days or longer to combat effectively. 

If you need to change the water during treatment add the equivalent amount of medication to the water you are replacing, to keep the medicine in the main tank at the right level. Have some aquarium salt on hand too as it can also be used as a medication for parasitic infection and fungal infections. It can even aid healthy fish after transportation.

How to set up a hospital tank for aquarium fish

Having a spare aquarium is always a good idea as it can be used as a quarantine tank to hold fish before going into the main display aquarium, but also a hospital tank if needed. Hospital tanks should be bare apart from a mature filter, an air stone heater, thermometer, and somewhere for the fish to hide, like a flowerpot. Fish can be removed from the main tank and treated in the hospital tank for specific ailments, but always treat the main tank too as the life cycle of some parasites may be in the gravel and on other fish, but not manifesting themselves. 

Always use medications as directed by the manufacturer in their product instructions.