Mg isn't that important unless you are doing SPS. You could hold off on getting the kit for now, but i would still check it from time to time just to be safe.
Phosphates are a different story and another can of worms. If you have an algae problem then it is good to test it. If your using tap water, which you shouldn't , then I would recommend testing for phosphates and switching to RO. There more phosphates you have, the more likely you are to have algae problems. Ideally you want them to be 0 but thats never the case.
so next time i go to the fish store im gunna get phos alk and possibly ca? and what do you guys feed your corals, lps,
Brine shrimp arent of much use imo... very, very low in nutriton... i really dont know why anyone uses brine shrimp... pointless really, but most likely of more benefit feeding to corals than to fish jmo...
woah! thanks afboundguy didnt expect that much hahaha k so alk ca and mg