low calcium levels

[nats]

I need some help please. I was doing my weekly water testing and my calcium levels have dropped down to 320 from 480 last week. I have a Red Sea Max 35 gallon. I do weekly water changes 15-20 % and use RO Di water. I am using instant ocean salt. The tank is three months old with two small clowns a frogspawn and 4 little zoa frags. Alk is low too. I was thinking of buying c-balance 1 & 2
Just wondering would this product or something simular help increase calcium levels or is it only going to help maintain calcium.
Thank you