Too much nitrogen? Ph off? Please help.

These were transplanted 5 days ago, didn't really have issues till I saw some mites a couple days ago, and hit them with azamax. I also fed them on that same day using Advanced nutrients half strength mild feeding. My ph meter had broken earlier so I had to use the ph test kit, so there's a good chance I screwed up the ph as well :/.
It looks like too much nitrogen/overwatering and possibly ph off? Any help is appreciated, I don't want to make problems worse by chasing issues that aren't there.

Inspect roots for pythium.

This is what the bottom looks like, do I need to rinse off all the cocoa to look at all of them?
They are all in 2 gallon pots with 30% hydrocorn/70% coco, the pebbles are on the bottom of the pots. They were watered with 800 ppm nutes. Air temps are around 80, sealed room at 55% humidity.

No you need H202(30%+ grade) in there , add it to the water every week. Looks like a diseased root system issue and the others will end up the same if nothing done.

What dilution/gallon should I be using with the h2o2? Would I be better off mixing coco and hydrocorn 50/50 vs hydrocorn on bottom with coco on top? I really don't want to repeat these mistakes.

50% grade @ 1000:1 35% @ 750:1

From now on just use coco mixed with perlite. Don't use clay pebbles on top as the coco cant dry between waterings and prob caused this issue.

The pebbles were actually added less than 12 hours ago. I'll remove them to help the drying process, but this issue occurred well before the addition of hydrocorn. Thanks a lot for the advice I'll be doing the h202 treatment tonight.