Gee thats an easy question to answer, NOT
it depends on a lot of factors, like tank size, calcium demand from bioload etc. There are few hard and fast rules but I will try to outline a few basic guidelines, and hope others will fill in the blanks.
On small tanks(say 30g and less) a good 2 part like B-Ionic or similar should work. A lot of people with nano reefs(20g or less) get by with frequent water changes only(prolly work for soft coral tank) Kalk seems to work well if you use 2 part or something to raise calcium and alkilinity to target levels and drip Kalk to maintain it. This can be pretty effective on tanks up to say 75g or so unless its a SPS tank or a clam tank. Even a large number of LPS corals can drive the calcium demand up to the point the 2 part isnt cost effective.
Big tanks with high calcium demand benefit from either a
calcium reactor or a
kalk reactor(like a Nilsen) and most reefers keeping large SPS/clam tanks use both a calcium reactor and supplemental kalk drip to keep parameters where they want. See that was simple huh, As a rule 2 part will buy you time to figure out your needs and go from there, its a matter of calculating costs like start up for a reactor vs long term buying 2 part additives. a side benefit to eather 2 part or calcium reactor is these combined with reasonable water changes should supply the trace elements needed eliminating the need to purchase seperate magnesium, iodine, stronium, etc, FWIW
; 2 Part products are easy to use but more expensive and Calcium Reactors are the way to go if you have a larger tank and the $$$.
