They're good to have on hand for a fast look at a situation, but never rely on them for water-balancing. I have freshwater and a pond, so they come in handy more often, but for maintaining a marine tank, a weekend testing of alkalinity at least (bellwether of other problems) will keep you out of trouble. It's the fastest test (I use Salifert) and if it's off, test magnesium, which is the fussier test) If it's off, first bring magnesium up to snuff, THEN the alk buffer, last of all the calcium. So you don't need to run every test every time if your water is in balance as I have in my sig line.