right now it is only calcium, alk, ph, and phosphate for me.
i think the helpfulness of the testing changes as the tank matures and the inhabitants change. now i have a lot of corals that are growing fast in a relatively small volume of water ~ 75gal. i also have a heavy fish load. as the mass of coral changes my calcuim consumption increases (hopefully). ph and alk are important for me to keep my calcium high.
i dont check Am, nitrite, nitrate unless something looks wrong. the corals are real good barometers for any changes that happen in the tank. if something looks odd then i will test nitrate. it has been 0 for a long time but there was a time in a smaller tank with a crappier skimmer that i would see an increase.
since i put a phosphate reactor on the system i rarely test for phosphate anymore.
of course i test salinity when mixing salt and for water changes. a water top-off almost eliminates the need to test the tank. monthly is fine for me. the refractometer is a good purchase.
I dose a homemade 2 part with a medical peristaltic pump, and put Kalk in the top-off water to support higher PH. calcium stays around 450, ph ranges from 7.8 to 8.1 and alk tends to climb ( i havnt figured this out yet) but i try to keep it around 11dkh. i add magnesium as needed, well there is another test. i test magnesium level every few weeks and adjust it then with whatever magnesium potion that is handy.
principally i believe that a tank benefits from heavy feedings, so i put a lot of food in (homemade combination of foods suitable for corals fish, and inverts). with a lot of rock, a sandbed, a lot of flow (3500gph in a 55gal), a good oversized skimmer, and inhabitants in enough variety and numbers to eat everything, i think a strong and stable ecosystem is built that keeps the tank balanced. i think my tank would consume a dead fish pretty fast with no effect to the am, no3, no4 balance.