Hmm..so I should rule out that adding boiled water or the heater being the culprit. Malerubybard-I'm using an overhead filtering system with no rain pipe, therefore the water has slight current but no bubbles to encourage oxygenation. It is true that I feed the discus with frozen blood worm(contains much fat/protein ???) and beef heart. Clothahump-I'm using super absorbent sponge to do just that, but I hope to eradicate the whole problem than labouring in that way everynight and scaring the discus too often. Arcab4/Colesea-I just bought a sponge filter today, with the intention to increase the oxygen content in the tank for the benefit of my discus since they are a little quick in their breathing, and I think I'm unknowingly striking on the solution of breaking that 'oily' buildup. Thanks, you guys offer great help and information.