The system has been mildly salted, but those crystals actually predate any salting I've done. I'm not sure what they come from and would love some insight if anyone has any.

One thought - my underwater pumps use hose-clamps. Could having anything metallic within the aquarium cause problems?

Another thought - the limestone additive I've used is sometimes applied directly to the growbeds (dissolved in water, of course). Maybe that's what it is?