What other fish are in the tank?
How long have you had the fish for?
How long has the tank been set up for?
What are the tank dimensions (length x width x height)?
What is the ammonia, nitrite and nitrate of the water?
How often do you do water changes and how much do you change?
What sort of filter is on the tank?
How often and how do you clean the filter?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is, will determine what fish you should keep.
Angelfish, most tetras, barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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I would do a 75% water change and gravel clean each day for a week.
Make sure the fish has compatible tank mates and suitable water chemistry.
Possibly isolate the fish so it doesn't keep getting attacked.