If your magnesium is above 1200 (1350 is a good number) and your alk is around 8.3 and cal at 420, everything 'locks' in place so long as you have a source of calcium, and if you have no stony coral or clams using it up, it can stay steady for a bit: testing your alk will tell you the need to test the other two, as it will not fall until the mg (which is used more slowly) starts to fall. If you do have stonies and clams, putting calcium (kalk) into your topoff water (1-2 tsp per gallon) will supply calcium as used up and keep the other numbers stable, again, until the mg runs down. I run without a controller or dosers, and just add kalk to my ro/di, plus water changes. I use Salifert tests and Kent cal, buffer, and mg.