I have 8 tanks. Only one of my tanks gets this oily looking surface stuff. I know it's probably protein, but why only one tank? The tank in this room now is new and the one I had before is gone, so same room different tank same problem. Here's some specifics: planted, discus tank, 82.5 temp, all parameters good, 56 gall tall, 98 watts lighting, flourish excel every other day, flourite substrate, wood, lace rock. Is it the pants, the rock, the flourish?