Oh Kalkwater is a calcium additive. You buy it in powder form and mix it with your top off water it adds to your calcium, but it also adds to your PH and ALK.
So you have to add it slowly and preferably at night because the PH is lower with the lights off.
You need to mix it then wait for it to settle and then only add the clear liquid from the middle. I use a small tank with a hole about 2" up from the bottom and have a small piece of airline and a small valve so I can just drip it in at night.
IO is low on CA and MG to start with so you will have to top it up if you want corals or inverts.
There are other ways to add CA but I am partial to Kalkwater.
Just remember don't add anything you don't test for. 
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150 Gal system 3'x3' 400W M/H, Bekett skimmer, Dart return,1/4 HP Chiller
180 Gal Drop tank, LED lights, Bubble master 250 skimmer,Hammerhead on a closed loop, Speed wave return.
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