What are the ambient temps in the room like? Do you heat a whole room or just the rack up?

I might be wrong but isn't scale rot caused by wet or prolonged damp conditions? So if you didn't have substrate on a portion on the tub that seems like it would lend itself away from scale rot not towards it. (depending on your humidity)
I would be more concerned with burns or mites, but if your sure your hotspot was 88 it seems burns would be unlikely as well. Do the snakes still have the little red spots?

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