Can you post a picture of the fish?
Check the fish for small white spots on the body or fins, and if there are no white spots, turn the tank lights out and shine a torch on the fish and look for a reflective gold sheen. The white spots would be Ichthyophthirius and the gold sheen would be velvet (Oodinium).
If there are no white spots or gold sheen, look for a cream or grey patches on the body. These will be protozoan parasites like Chilodonella, Costia & Trichodina.
You can treat whitespot and velvet with heat. You get the tank temperature up to 30C (86F) and keep it there for 2-3 weeks, then bring it back down.
Do a big (75%) water change and complete gravel clean before treating any tank, and clean the filter and wipe the inside glass down. Increase aeration/ surface turbulence to maximise oxygen. Warm water and water with chemicals in holds less oxygen than cool chemical free water.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of the page.
Remove carbon from filters before treating the fish.
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The SeaChem ParaGuard will treat all the above mentioned parasites but heat is a safer option for whitespot and velvet.
ParaGuard contains Malachite Green, which is toxic so wash your hands with warm soapy water after handling the medication or working in the tank. And keep the medication away from children and animals.
