I've not had much luck with my Tunze Osmolator pumps. I've gone through two of them. My check valve is fine and no Kalk has been drawn back into the tank.

I managed to free up a pump by using WD40.

If any of this residue gets into the reef tank, what would happen?
My guess is it would get skimmed, and I've heard it is a form of fish oil. Anyone have any experience with that?