Reef Crystals. Salts are going to fluctuate from batch to batch, period! I toss in a few tablespoons of SeaChem Mag (I always buy the largest amount, it comes in a small bucket) in with my water change. I guess I don't test much anymore, just keep my eye on the animals. I do make it a point to do a 35g water change every week though and my water volume is somewhere around 500g, so if your smaller throw in a couple of teaspoons of mag with any salt if your having a problem. Getting Mag to increase is tough, it takes a ton to get it up to 1350 plus. I was way low and it took weeks of dosing to get it back up. Go slow, it's not a race.