I don't see a need for you to dose Mg and Ca as the organisms you keep are not fast consumers of these elements. Fast consumers of such elements would be clams, sps, coralline algae. Dosing additives can be done as long as you follow the dosage instructions. If you want to avoid the hassle and cost of such additives just change more water per week, maybe even up to 20-30%. This is the advantage of small tanks. You can maintain your parameters just by regular water changes. Instead of adjusting each parameter at a time, changing water will set things right for most of the parameters.