Hi Kurt, I run into this every year. Sometimes tc starts of Colocasia just have a problem. My trials with spider mites and fungus gnats don't help. Cool nights definitely don't help!!
My guess is you have a tuber/roots problem and fertilizer and water won't help. Heat may! What I would do is separate these plants into small (3 or 4 inch) pots, checking the roots as you do so. One may be infected while the others aren't, or are less so. I use a well draining mix for rehab projects even with baby Colocasia.
Cool nights won't help so if you have a spot that stays warmer at night but won't fry them during the day, great. I've had some mixed success putting a one gallon milk bottle "cloche" on them at night.
Good luck!
