How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the water?
Basic first aid for a fish when you don't know what is wrong:
Test the water quality for ammonia, nitrite, nitrate and pH.
Wipe the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for a week or until you know what the problem is.
Make sure any new water is free of chlorine/ chloramine before it's 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. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase aeration/ surface turbulence to maximise the oxygen in the water.
Start a thread and add some pictures and video of the fish, and a full description of its symptoms.
