I ran into the same issue you had when I was in the first few months of the tank. I used Reef Crystals, and didn't test for Ca
. Coralline algae suddenly started fading. Tested for Ca
and found levels at 320 or so. Not good.
What I did, and would recommend since your alkalinity levels are OK...
Use TurboCalcium (or any quality calcium chloride) to slowly raise the ca
level in your main tank to where you want it. I wouldn't raise it more than 20ppm or so a day. Use the calculator previously linked to figure out how much. Mix it outside the tank with RO
water and slowly dribble it in a high flow area.
Then, once your Ca
levels are OK, start doing a 2-part solution dose using equal parts A and B. You'll have to test every few days or so for several weeks to determine if you're adding the correct amount, but once you get it figured out it's pretty simple. I've got a 46g and use 20 ml of each A and B (C-balance) every morning before the lights come on.
You'll also have to bump up the Ca
levels of your PWC
water if you continue to use Reef Crystals. I do this by adding a solution of TurboCalcium to my water change bucket just before I do the change.
Those articles linked, as others have said, are the Holy Grail of Ca
articles in my opinion.