Dosing

You are on the cusp of needing to dose. Unless your are getting ready to buy a ton of coral I would look into Kalkwasser and using some with your topoff water. The first thing that starts to slip is alk. Usually below 7 is lower than ideal. When using Kalkwasser it will also benefit your calcium. Magnesium is important but at the lower levels of consumption you are experiencing right now your water changes will keep it in line.

If you change your water once a week try starting out by dripping 1/4-1/2 gallon of lightly saturated limewater into the tank either 3 or 4 days after your water changes. It is the cheapest and easiest solution to hold you over. The reason I suggest this instead of 2 part is you might find yourself wanting a calcium reactor once you learn more and I'd hate to invest in dosing pumps ect before you weighed all your options.