I have noticed I am getting a bit of calcium buildup on the return tube from my sump to the tank. I am using 1/2" plastic tubing with the aluminum bands in it to give it strength. But it seems this is the only place I am getting it in. I mean the tube pretty much goes straight, takes a 's' shaped bend, goes straight and the does a 'u' into the tank over like 3'. And it only seems to happen in the lower bit. Anybody know why?