It depends on what you need from it. If you just have fish then almost anything will work. If you have a reef there are two methods. One mimics natural seawater, the other has additional supplementation to keep levels high enough in our closed systems. I like to have supplemental levels since in our closed systems certain things can be used up quickly and then you have to supplement. I would rather have enough to last the week.
I use Tropic Marin, regular not pro-reef. Apparently the pro-reef was designed for higher end systems with calcium reactors or automatic dosers and therefore has less Ca
, etc. Even they say they should have switched the names. I like the Tropic Marin because it is all pharmaceutical grade ingredients and is very consistant from batch to batch.