Hi Joe,
I agree with hoppy. My recommendation is to throw away your PO4 test kit as well as your NO3 test kit. Yor ammonia and nitrite kist also have limited use. It's very doubtful that you have zero ammonia or zero nitite. The zero reading simply means that the reagents are not capable of reacting to concentrations below a certain value, that's all. The job of those two kits is to advise you if the ammonia/nitrie levels are toxic to fauna. They cannot measure below toxic levels.

Any tank with living things in it has ammonia and nitite. The nitrifying bacteria depend on the existence of these waste products so it is not possible that the values are zero, otherwise the bacteria would die.

EI ensures that you have at least enough of everything so there is no need to measure these values at all. Take the money you save from not buying these kits and buy more plants instead. Use the time you save by not having to do all those flawed tests to relax, have a beer and to enjoy looking at your tank.

Cheers,