Is it fin rot or tail bitting

[Laki]

His living space is 1.75g approx. his fins look pretty bad. He needs to be separated and in some clean water changed twice a week to improve. The red is probably ammonia burns. A 2.5 should be cleaned twice a week with ONE fish so you are probably dealing with serious ammonia poisoning there.
You will need to step up water changes to every second day as long as the 2 fish are in there.