Help required from the more experienced PLEASE

[ozzyoscar]

Hey All
I'm new around here, and registered in the hope of getting some advice on my common pleco. I have 2 oscars (about 5") and what were 2 identically sized common plecos of about 4" in an 80 gallon tank. About 4/5 weeks ago, the plecos seemed to be getting a bit testy with each other, one chasing the other off the food I placed in for them. I always put two lots in, one at either end of the tank to avoid such squabbles.(I realise that I'll be up for a bigger tank to house them both in time.)
Anyway, they have stopped annoying each other now, but in the ensuing 4/5 week period, one has almost doubled in size and is a voracious eater, the other has stayed small and much darker (they were the same) and unfortunately has clouded eyes. When he sits on the bottom or on a root, all his fins are down (where his mates are always up and looking splendid) and his appetite seems to be well below the others. He will lob himself onto a lump of pumpkin/zucchini or an algae wafer but moves off it shortly there after, whereas his mate just stays till it's all gone !!
I haven't noticed any fighting between them at all, they'll happily eat the same lump of food now, so I don't think it's territorial.The Oscars don't bother them at all. My water quality is perfect, checked very regularly and changed religiously.
My aquarium guru gave me some "PHENOXINE" to treat the water - made up of (for you technos out there): 2phenoxyethanol 17.6ml/l, formaldehyde 37% 28ml/l, brilliant green 0.24g/l and malachite green 0.08g/l- and I have treated with this twice over the week to notice no change. Perhaps, just maybe, he is a llittle more active than he was. That medication is a broad spectrum one designed to treat white spot, fin and tail rot, slimy skin desease and body/mouth fungus.
Sorry to be long winded, but I think I've covered everything - I'd just love to be able to get this little guy back on track, but sure could use some pointers.
Thanks in advance.

[Plec0maniac]

hi the bigger plec is more dominant than the other plec. that is the reason why the one isnt that active, bec. of the fact that the bigger one is very territorial. Oscars tend to be aggresive so be careful! Just try to separate the smaller one..