Depends on what you are keeping in it. For just fish really nothing, for softies, lps, I would just keep track of calcium and mag. levels for dosing. For SPS and/or more difficult corals you will need a full range of tests and probably would want to dose kalkwasser to keep calcium and alk correct. I keep mushrooms, leathers, lps and dose calcium very rarely. My salt mix (reef crystals) seems to keep it up very well with weekly water changes but I had to dose often when using Instant Ocean as it has less.
Phosphates tests almost always test zero unless you have a lot because algea uses it all up so make sure you buy a quality test for that, I use sailfert. Anything dosed should be tested but honestly for the corals you are keeping so far I would just keep up on water changes with a good salt mix and keep an eye on nitrates, calcium, alk, ph and maybe mag. Don't know about that one as I don't test for it.