How much or how often do you feed? And what?
This is, believe it or not, generally related to mysterious levels of NO3/PO4. More food = more waste (both food and poo).
Most people believe they do not overfeed, and that there has to be some crazy thing happening to the tank.
Try reducing your feeding by say, 25%. Watch your levels and see if there is any change. 25% reduction in food won't hurt your fish and its a no risk trial.
I don't think RO has anything to do with it. I don't run RO (NEVER have) and I never have any NO3, NO2, PO4.
Go Tapwater, tapwater, tapwater woooo! (use any rhythm while singing
