Hi and welcome to the forum
How long has the tank been set up for?
I assume the water quality is good with 0 ammonia, 0 nitrite, nitrate less than 25ppm and pH around 7.0?
How often do you do water changes and gravel cleans, and how much water do you change?
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The fish appears to have excess mucous or possibly fungus on its side. The area under the lump looks red and that is either bacterial or protozoan. The fish might have been injured and developed a bacterial and fungal infection in the wound, or it might have a protozoan infection on that area.
Do any of the other fishes have cream, white or grey patches on their body or fins?
Are any of the fish rubbing against objects in the tank?
If yes to either of these questions, the fish have a protozoan infection like Costia, Chilodonella or Trichodina.
These can be easily treated with any broad spectrum medication containing Malachite Green. However, because you have catfish in the tank you should only use the medication at half strength or you could kill the catfish.
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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.
Before you treat the tank, wipe the inside of the glass, do a 75% water change and complete gravel clean, and clean the filter. If you have carbon in the filter, remove the carbon and throw it away.
