Are LEGO elements safe to use in an aquarium with fish? I've never seen LEGO elements in a working fishtank.
The main restaurant in the LEGOLAND park in Windsor has quite a lot of LEGO creations in fishtanks. In the same park, the Sealife attraction 'Atlantis Submarine Voyage' has 50 species of sharks, rays and tropical fish that happily coexist with the LEGO models.
Keep in mind that LEGO is safe from children saliva, which I'm willing to bet is more aggressive than fishtank water.
LEGO bricks should be safe for fish. Avoid using small parts that they could mistake for food. You also might be cautious on the paint, but I'm sure it's still safer than the paint used on the cheap stuff you'll find at fish stores.