Oto cat mystery affliction

[Cunnimeg]

Hey all!
I am a first time aquarium owner and I'm struggling to identify what is ailing my oto cat.
At first I though he had ick, as there were two or three small white bumps on his side. I decided to try the heat treatment on account of my rcs and no hospital tank. 1 week later he is very lethargic and slim. White spots are gone, but he has a clear blueish film coming off of him. It looks like he's shedding skin almost. I now suspect some sort of fungal infection maybe? This evening I've dropped the temperature in my tank to 80 (86 before for heat treatment). All other fish in the tank are thriving. I suspect the little guy won't pull through, but I'd like to know what I'm dealing with!

Stock:
I have plants, 4 harlequin rasbora, 5 cardinal tetras, approx. 20 rcs, and a plethora of snails.

Water parameters:
Addmitadely not ideal, I believe on account of a almond leaf that was left in for a while.
Ammonia <.25 ppm
Nitrite <.25 ppm
Nitrate <5.0ppm

I vacuum and do a 20% water change every Monday.

The shrimp were added 2 weeks ago. Purchased from a reputable breeder.

 

[Cunnimeg]

Any insight?

 

[Delapool]

Was there any flashing at all?

 

[Cunnimeg]

Was there any flashing at all?

Not that I've witnessed. He did spend a little bit of time at the top of the tank yesterday however. Mostly he stays very stationary, usually close to the air stone bubbles.

 

[Delapool]

Makes me wonder on a gill issue. Could any parasites have come in from 2 weeks ago? Eg even something like velvet which could explain the shedding skin (excess slime coat maybe??). Else would try fungal or a general cure.