Are his scales sticking out from his body, around his belly?
If his scales are sticking out he has an internal bacterial infection and these can be very hard to treat.
If his scales are not sticking out from his body he could have a protozoan infection or an external bacterial infection.
A protozoan infection will cause the fish to rub on objects and the fish normally gets a cream, white or grey film over the infected area. A bacterial infection will make the area red.
The fish in the images look like it has a bacterial infection.
You can try a broad spectrum fish medication, however if it contains Methylene Blue it will wipe out the filter bacteria. Try not to use anti-biotics because they will wipe out the filter bacteria too.
If you can't get a medication that doesn't wipe out filter bacteria, then try to treat the fish in a separate container (40 litre plastic storage crates work well). Fill the container with clean water that is free of chlorine/ chloramine and aerate it. Put the sick fish in it and treat the fish in the container.
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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 this page that will let you convert litres to gallons if you need it.
Remove carbon (black granulates substance) from the filter before treating or it will absorb the medication and stop it working.
If you treat the main tank, wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.