Originally Posted by Farslayer
Skimming can't pull out elements like that, your LFS guy is wrong. Skimmers remove floating particles in the water which will eventually decompose into nitrate. The bubbles they create will have this gunk on their surface based on a nice little law of physics. The bubbles move up and pop, spilling the junk on their surface into the collection cup. Elements and minerals are dissolved and would not be on the surface of a bubble, so he's just wrong about that.
Actually, skimmers do remove some vital elements, but hopefully not at a rate faster than they can be replaced through water changes. Their benefits are far outweighed by this, however.