Glad to hear you got it figured out. Too bad though that your tank ended up so low. I don't have any experience with low magnesium, but I would think that for the short length of time it's been low, you shouldn't have any problem.
In the process of raising it, I'd personally go the slow route and only raise it 10-15 ppm
per day. And I'm pretty sure Seachem, or your LFS
, will refund you for that bucket of salt. Either that, or they'll give you the "hmm... y'know you're supposed to remix all that salt up in the bucket before use because it settles into layers during shipment" speech. And no... I never do that.
Ever since Reef Crystals recently had that supposedly weird batch with low Ca
, I've always done a complete series of tests of a sample of water made from the first scoops from a new bucket. It's just good piece of mind, and an easy way to catch something quick if I happen to get a "bad" bucket.