Needing help with my salt levels - keeps going up

[ashr]

Hey all

So some background first. 66g red sea max. heavy SPS and LPS corals.

Id like my levels around 1.026 but found it was around 1.030 today when I checked. I only top off with fresh R/O water but sometimes the top off is around 2-5 gallons. I do a water change every month (I know I know but this works well for my tank) Whenever I do a water change I only add water with a salt level of 1.015 to 1.020 to help lower it in the main tank but it only goes down a bit once I add it but it always creeps back up..

Any ideas why?

I dose Mg, Calc and Alk.


Thanks!