Emaciated fish that eat but still lose weight (or chew food and spit it out) can be a sign of internal parasites. Have you seen the fish poop? White stringy poo would also indicate this.
Try treating with an anti-internal parasite medication, medicated fish food if you can find it, and see if this helps.
Edit: It's NOT ich!!! Blimey. Ich is characterised by small white spots covering the fish's body and fins. It is a parasite but an external one.
