Seachem products perplexing

[Myka]

First things first, how big is your tank and what corals do you keep in it?

The Reef Calcium is liquid. Reef Complete is also liquid, and includes a bit of magnesium and strontium as well. Reef Advantage is the dry version of Reef Complete (my pick of the bunch for calcium).

Reef Buffer is sodium carbonate and will raise pH. Reef Builder is sodium bicarbonate and won't raise pH. I would pick the latter.

Reef Fusion is a 2-part additive. It just makes dosing easy because you use equal amounts of each. Part 1 is calcium and some other minerals, part 2 is carbonates (for alkalinity).