Welcome. GFO and phosguard are different animals. GFO is best in a reactor as to keep it from clumping up. Phosguard comes in a little bag. You just dump it in. It also removes silicates which is nice in a new tank to lessen the diatom algae burden. Since you’re getting such a low reading in a new tank, you shouldn’t even touch it. If you’re keeping corals, it’s dangerous to get too low. If you have fish only, there’s really no need to worry. But just keep testing it.