If the muscle tissue is turning white it could have spironucleus. The only way to treat this is with high levels of salt.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate. 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. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
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Add 2 heaped tablespoons of rock salt, sea salt, or swimming pool salt for every 20 litres (5 gallons) of tank water. Keep the salt in the tank for 4 weeks.
If there's no improvement after a week of salt, add 2 more heaped tablespoons so there is a total of 4 heaped tablespoons of salt per 20 litres of water.
When you do water changes while using salt, add salt to the new water before adding it to the tank. This will help keep the salinity (salt level) in the tank stable.
Watch the catfish if you increase the salinity to 4 heaped tablespoons and do a water change if they start to stress or freak out.