You can also use vinegar and a razor blade to remove the calcium deposits from the water line.
acid still works better hands down, used 10 year old saltwater tank from someone on craigslist, had it down to mint condition in less than 30 min, acid literally melts it away. just rinse it off after, no scrubbing needed. very harsh if u accidently inhale it though. takes ur breath away. i used it at work to get barnicles and rust of of boat propellers. works wonders im telling you. just dont put it on aluminum and let it sit. works very well for anything protein based like calcium build up. takes rust off steel like nothing too