classic sea salt mix

I am new to Saltwater. I do not have any corals. I have 2 clownfish. Redsea reefer 425. I keep putting in Alkalinity, Cal, and MG thru apex DOS. The levels keep dropping. I do an automatic water change of 1.5 gals every day. I use Classic Sea Salt Mix. Alkalinity - 9-10 dKH (approx) Calcium...