[trilinearmipmap]

I've never had any problems with the 2-part green and white epoxy they sell at J & L and I'm pretty sloppy about using it in the tank without paying attention to carbon etc. If that is the kind you used I doubt it is the problem particularly since mushrooms are hardy and don't care much about water quality.

I would consider whether too much light is the culprit particularly if there was any change in the height of your lights off the water, my mushrooms hate light. The other thing I would consider is a contaminant in your RO water and I'd consider testing the RO water for TDS. Not that increased TDS itself would hurt the shrooms but it could indicate that something else such as metals etc could be leaking through.