I tried this on Moparts General, no luck.

I just picked up a heated, water based, parts cleaner at auction, it had been used in a factory to clean machined parts. Don't know why it is bothering me, but the tank is coated with some hard gunk.

Let me add a little. The washing tank is plastic and has splotches(almost complete coverage)of a hard gray film. It is almost like paint in thickness. In a few spots where I could get a scraper under it, it popped off. This doesn't mean it peeled off. So far I have tried Simple Green, Purple Power, wd-40, paint remover, acetone, lacquer thinner and PB Blaster.

Like I said, it was used in a factory and had plenty of metal chips in it. This might give someone a hint. I think it had the microbe/detergent in it before.