i use to check every week but i stoped, i pretty much follow a schedule with my tank with water change every week, carbon and phosban media change every 2 weeks, twice a day alk and bionc twice a day till i reach 450 . i only check cal till i reach that point.,then check every 2 days.
I test my water once a week and I also watch the corals and look for changes. I know when my Zoas and toadstool start to grow that my water is probrably on the dirtier side.
My SPS and certain Zoas grow great when the water is clean, When I let the Nitrates get around 20 then they slow down and everything else starts growing. Doing a 25 gallon water change this weekend.
It's from having more waste from the fish in the water. Zoas also filter feed, not the same as a clam or sponge would but they still do. And most softies absorb the water right into it through the flesh and they pull out organic material that way.