My Sea Chem bucket was also very low in Magnesium. It was low in everything else, but not nearly as bad as the magnesium. I don't like how it leaves a nasty debris film on everything in the mixing container (brown calcium deposits....yuck!) My tank is looking better, but I don't know if it's from the Sea Chem or not using RSCP. My SeaChem bucket wasn't acceptable on any level...alk, ca, or mag. But...it was higher than other salts I've been using lately (see my Great Salt project results.)