I have an idea about your mystery froth. Hygrozyme. Are you using it? I've been pondering your res, and I remembered people complaining that hygrozyme turned their DWC reservoirs into brown frothy messes, similar to what you had going on. So... my deducing makes me think you are using hygrozyme or some enzyme, and it became frothed up by either airstones or a water pump in the reservoir. I think it would explain it not coming back, too, though I bet it would at the next reservoir change.
Am I on to something, or on something?
Am I on to something, or on something?