Ever used Krylon Fusion- under the water line?
 

I'm planning a couple foam backgrounds, and I've seen a lot of reports that it's safe, and not a single fish death linked to it submersed... Still...

Has anyone here ever tried it?

Not yet, but I've read/heard the same thing, so I have painted some PVC pipe with it that will be in my 125 gal. So it will be a few weeks before I really test it.

Im thinking about using it to coat a foam background with a little water pump that pumps water over "shelves" for emersed plants, and ending in a little waterfall....

It seems cheaper and easier than dry-lok or epoxy...