If you are using high light and co2 then you'll absolutely need the following:
Calcium (api test works even though it says salt water)
Iron (I suggest the Hanna test kit as the api ones don't seem to work)
My gh and kh are around 8-13 on most days depending on when I changed my water. The important part is not that they are that high, but that they are in the correct proportion of calcium and magnesium for the gh: https://www.plantedtank.net/forums/1...g-calcium.html
If you look at the dosing threads for high tech plants in this same sub forum, you will see that people are aiming for 20ppm of nitrate and 5ppm of phosphate with 30ppm of co2 with high light (60+ par at tank floor). People usually don't test for it, but you'll need around 15-20ppm of potassium too. However, that already comes form potassium nitrate (for 20ppm of nitrate) and mono potassium phosphate (for 5ppm phosphate). However, you'll need to dose potassium sulfate if you skimp on either of those, say if your tank already prodcues enough nitrates from fish waste alone.
If all of this is over your head and you are running a low light, non co2 tank, then I'd still recommend everything above because at some point you are going to have issues and will need to test everything to see where you ppm's stand. Tanks don't live forever without human intervention, but low light tanks can go a very long time without major issues besides diatoms here and there.
Finally, use this site to figure out how much to dose to hit X ppm: https://rotalabutterfly.com/nutrient-calculator.php
. Just be aware that this site will think you need to reach say, 5ppm of nitrate instead of 20ppm, so you need to multiply accordingly. Low light? The targets at this site are fine then.