so driving to mcdonalds yesterday i spied a 55 gallon tank and stand sitting on the side of the road with a "FREE NO LEAKS" sign on it. sweet! it even had a bag with an aquaclear filter and some brushes and stuff inside of it. so of course i loaded it into/on top of the car and took it home. it was (and hopefully will be) a really nice tank that definately has been neglected. i cleaned it as best i could but the top three inches of the tank have this crusty stuff ( i think calcium) where the water level dropped for an extend period of time. I've scrubbed and scrubbed but its still there. never really had any experience with this. any tips for getting rid of it without harming the future fish i'm gonna put in there?