Have you checked the rain water for heavy metals or contaminants?
Rain water quality is good.
Could it be I clean he tank too much?
I had taken all the water out of the sump after stirring the bio balls and porcelain rings.
I did a water test the next day - ammonia was approx. 0.25 so I took 50% water out of the tank.
What is your regular water changing schedule? I do about 40% weekly.
Meth. Blue is ok to add, just be aware that it can stain almost anything so you'd not want to spill or drip any of it and a little bit of it goes a long way, but the dosage instructions should be on the bottle.
Approx. 1.5 weeks to 2 weeks depending on water test results.
I am still monitoring.
Black Molly still has the silver thing but she remains active.
That's definitely not too often for water changes. I would still recommend Prime since some ammonia did show up, it will deal with heavy metals that you can't test for and that can be present in rainwater and it will provide slime coating enhancement for your fish.
One of my black mollies had it and didn't have any problems. It appeared and disappeared for unknown reasons!