there a lot more things that absorb alk and ca live rock is one of them and coralline algae is another but I would be careful doing a water change with salt water mix that has a high dkh may not be a good thing for the long term, I don't think you are taking everything into consideration the cost of alk and ca is pennies on the dollar compared to what you have invested in the live stock don't cut your nose of to spite your face you need to look at the big picture