Good call on testing nitrates regularly. Especially if you are feeding a lot they can spike. 20% weekly water changes is probably what's keeping your majors (CA, Alk, MG) in check. If it were me though I would not wait until I see Coral Health starting to decline to start testing. I would buy either a Red Sea Foundation Pro or set of Salifert test kits and test the major 3 weekly if not bi-weekly. It's definitely a proactive not a reactive thing with keeping those levels balanced. Sounds like a nice set up by the way. I also use natural seawater in my tank but mine is not fortified so I add elements on my own, testing regularly and adding what's needed. It wouldn't hurt to also test phosphate levels. That's a pretty important one too.