I've noticed a lot of hard water buildup/deposits along the top rim of my tanks, where the water level is, which makes sense, but also in weird streaky patterns inside the tank walls. 1. Why do I get the deposits inside on the walls? I scrub the inside walls occasionally with a sponge when I clean the tanks, but that's usually to remove the streaky deposits I see. 2. Will the removal of it with a sponge harm my fish? They all seen fine, but I does cloud the water for a few minutes when I clean it off. Stirs everything up, much like gravel vacuuming does, and then it all settles again.