Yeah, I mean check under the hydroton.

PH is a possibility, so is overfeeding, but I'd check the roots first. Then PH, if its overfeeding it will fix itself while its on the rinse cycle this week. I'm not completely sure the difference between food grade 202 and peroxide. I've always used peroxide, works wonders for me. Doesn't sound like an O2 problem, even if your stones were a little clogged, clearly there is enough air in the line.