Dying SPS frags

[MalHavoc]

How long has the tank been up? Many new tanks have difficulty keeping SPS corals alive due to immature water conditions.

You may want to increase your tank circulation. Many (not all) SPS corals live in natural environments where they get pounded by hard current swells, and if they do not experience these sorts of things in the aquarium, they can suffocate. Montipora isn't normally an SPS coral that requires huge pounding currents, but I question whether you're giving your frags the current they need. I'd aim for > 20x the tank volume in total circulation, per hour, and even more if you can swing it.

Have you measured your phosphates? phosphate levels that are detectable can impede calcification and prevent the growth of stony corals.

If you're worried about your nitrate, I'd either plumb in a refugium, add a deep sand bed some place, cut back on your feeding, or upgrade your skimmer. You're not using tap water, are you?