Any chance of some pictures of the fish from the side?
Is the fish eating ok and acting normally?
How long have you had the fish for?
How long has it been red like that?
Have you added anything to the tank in the last 2 weeks?
Can you post some pictures of the other fish in the tank (side and top views)?
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Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn.
Increase surface turbulence/ aeration o maximise the oxygen levels in the water.
