Originally Posted by schpanky
1+above comment. IMO
this is perfectly normal, just part of having an aquarium. I've always referred to it as calcium. People recommend cleaning it with vinegar (of course not pouring it in the tank).
I used to clean it with pledge (the dust remover) and had great results (once again, not in the tank). Not sure why it works.
The great thing about lemon juice, vinegar, or a citric acid solution is that if you get a bit in the tank you'll probably be ok. You can even feed orange pieces to goldfish* so citric acid and lemon would be quite safe as long as you didn't go squirting some into the tank.
On the other hand some of the stuff in those household cleaners can be really nasty to aquatic life. On the outside of the tank you'd be safe as long as there was no overspray or applied directly to the cloth, but still... it would feel significantly more risky to me.
*(Don't know how other species would react to citrus.)