Medium problem killing Phals; B4 I repot knowing cause will help

[Skippersmom]

Greetings all wise and all knowing 'orchidistas'!
I live in Nebraska, so 'All My Children' are living indoors, sitting on humidity trays and with occasional warm mist humidity. Watered weekly with about 20 minute dunks in the sink with 3 of their closest friends. Alternate fertilizer, kelp or probiotic.

So,... meet the 4 members of the Aquatics 101 program. Fresh out of their pots as of last night. 'Look Ma! No roots!" [insert favorite explicative].

Back story - they are part of a group of about 11 Phals I rescued from my Dad's place after he went to long term care. They were in various states of too dry, too wet, never been repotted, etc. Some with poor roots. They all got repotted. Some right away. Some I cleaned up the bottoms, put in water [cuz I didn't know better]. Most of them sprouted some baby roots. Last fall, everyone got potted, [you may have seen my catt question in the fall - it got potted at the same time, same medium, etc. it's doing great!] I used a phal mix from 'RePotMe' and I often pull in just a little moss because in the winter it gets so dry, I like a few little pieces of something that capture a little moisture.

So all these newly potted babies are doing great from October until about a month ago! Plump leaves!New growth! I was getting ready to give a few to my siblings as a memento! Suddenly leaves are yellowing, dropping or shriveling. The medium starts looking like its dusted with charcoal. The moss is no longer yellow but has turned bluish gray. FREAK OUT.

I dumped 2 pots last night and discovered what I feared, roots are gone, the stem is blackish. Carefully gave a long tepid rinse and then spritzed w/ 3% H2O2. Each got plopped into separate glass of water while I reconnoiter. Then I took a couple plants that still looked good but had sketch looking medium. Underneath it all, the medium looked bad and the roots can be salvaged.

So far nothing like this in other Phals that are sitting in pure moss. [Potted the year before - spent the summer outside and were heavily watered but still happy! ] Nothing like this in my other pots. The medium was new and had been in sealed bags but in my garage [temp changes in the summer]. It smelled fine and looked fine when I potted. I am wondering if mold or schmutz from the humidity trays got up into these pots? Has this happened to other people? This is the first year I've used the humidity trays.

Should I consider spraying down the mediums of all remaining orchids with my antifungal [I have the good one ya'all recommend], several times to treat? Was this an overwatering issue only?[miscalculation of having humidity now after wintering orchids in my own personal Sahara for years?]

Your wisdom appreciated.
Pictures are of the swim class, medium of one of my pots that is still intact with a still healthy looking Phal and moss from another Phal pot] I think I should yank and re-pot before things progress 'underground'.