Sick Glass Catfish

My cat classfish is sick. He has white patches on him. I’m Guessing he ate a dead neon tetra that had similar symptoms which I can’t find the body for. I have separated the glass catfish out the tank. Is there a way to save him or should I euthanize? If I have to euthanize, what’s the best way to do so?

Ammonia: 0
Nitrite: 0
Nitrate 10 ppm

Microsporidian infection or muscle wasting disease.

Do a 75-90% water change and gravel clean the substrate.
Make sure the new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter. Wash filter media/ materials in a bucket of tank water and re-use the media. tip the bucket of dirty water on the lawn/ garden.

Add 2 heaped tablespoons of rock salt for every 20 litres of tank water. Keep salt in the tank for 2-4 weeks.

Let us know how you get on please, this is one of the tricky ones to cure.