There are test kits for phosphates and iron. In my opinion, its not worth testing iron as I've tried and never got good results and it doesn't stay long in the water column according to people. Potassium test kits are very pricey so most do not test it. Most sources of water have very little to no potassium.
When dosing ferts, a lot on here dose EI. In EI you add ferts at a quantity where it is not the limiting factor in plant growth and do a big water change each week to reset the level. There is also another called PPS which you add a minimal quantity. Some have their own regiment. Best to review the threads on here to read the ideas and opinions to decide which would work best for you.
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