Might interest you: DIY RDWC Design Question - #37 by Nitt
Take that back cover off of your pump(s). Also, take everything else off that doesn’t need to be there (eg: the rubber suction feet).
-That stuff in the bottom could be hydroton dust, or even your gypsum (stained/discolored) residue.
Probably hurt temps. The air pump generates heat, and the air it’s pumping into the water is as hot as the source (room it is in).
-If you have co2 supplementation, pumping that high co2 air into the water will affect the pH.
Some of this might’ve been mentioned. But I just speed read through and wanted to… share a couple thoughts.
If you’re still having any mold issues, or just want to give the room a “disinfecting”, maybe try an ozone generator. For a liquid “disinfecting” solution, you could try hypochlorous acid (HOCl). Pre-bought, or “made” with a generator. (I think I might’ve already seen you in some threads discussing HOCl).