There's a product called X-14 which is sort of a predone bleach solution you can spray on: keep it out of your tank.
Two other very good products are: --Dry-lok, I think it's called: goes on like paint and seals a masonry or plaster wall. Looks a lot like concrete when it dries. If you are getting moisture through a basement wall, this will stop it. It's even fish-safe: we used it to seal a planter [one of those old brick things inside the house] and turn it into a fish pond.
---Kilz. Comes in spray cans and by the bucket: if you have a persistent stain of any kind, including mold, after you have stopped the water that is causing it, ---it will cover it. It will also cover a wall so your next paint goes on in one coat, no matter WHAT color the previous paint was.