Soil Test Kits
You can try Soil testing kits in the Garden section at Homedepot, Lowes, etc.
Soil test kit includes pH, Nitrogen, Potassium, Phosphorus.
For iron there's a water test kit, but don't know if it's sensitive.
Calcium and magnesium are determined with GdH, but I think API might have a calcium specific kit for saltwater reef setup, same goes for other nutrients.
I would research the range specifics for your plant/fish requirements as you don't necessarily want to be stressed in being strict about maintaining specific (probably unrealistic) values in your tank.
I would definitely invest in a pwrhead and maintain weekly water changes to maintain circulation/dilution of nutrients.