White spots on back of tank...

[jasond]

Quick question. I have a 20G tank, been set up since beginning of April or so. I am in the process of having a bigger tank being built, so I have been holding off doing too much with this tank as it will become my sump. Currently not a whole lot for stock....about 25lbs of LR, GSP, mushroom, and one other I cant remember, and two clown fish...

Question is, I seem to get white spots on the back of the aquarium, almost like little crystals, and I have no idea what they are. The back is painted black, so they stick out fairly well. They will come off with the magnet, but just curious where they are coming from.

Water parameter's are good (ammonia, nitrate, etc). Temp is around 78, or as close as possible.

Any idea's?????