im not sure that if it was a bite wound that it would turn 'white or fluffy' over night. Imo its more likely a bacterial disease, in which case you should treat the whole tank with myxazin or similar anti-bacterial treatment. You could take the fish out and treat it but you may be left with the problem of the disease being left in the tank. Pictus cats can be very tempermental and delicate especially when young and would become affected by disease very quickly if left exposed to it. It would not supprise me if the pictus cats did take a bite out of the gourami as they can be quite nasty and territorial, especially with each other. In which case i would still treat with something similar as it would be the bacteria growing in/on the wound that would kill the fish! In all fairness dwarf gouramis never live long anyway, the interbreeding that is done to create all their fancy colour variations has weakened their immune system and are prone to a number of diseases and very rarely live a full and healthy life!
Good luck anyway,
David
