Hey guys my marble catfish hasn't been eating for a month, he eats once in a while but not much, he also swims against the glass and is rubbing his nose raw, i've offered him places to hide but he does't use them. He doesn't look sick or anything.
+1 that's my guess, check you water params and post please. In the meantime, do small frequent water changes, 10% or around, and keep the lights off. Build up some bigger hiding spots for him, I know catfish can squeeze in to stupid small spots (my bullhead and channel cat are testament to this) but it may help him relax a bit more. The lack of appetite and constant glass banging would point to water quality however.
If your concerned enough to post on a forum about your fishes health, why not spend the $20 and go get a new test kit and see whats going on. If your having a bad ammonia spike or have extreme nitrate levels, 10% WC isn't going to fix the underlying issue. Of course do small frequent water changes in the mean time but testing your water is the only way to confirm/rule out water issues.