Phosphate chemical kits will often give you incorrect readings when testing RO water as it is ionically pure and without a buffer system.

The reagents rely on the water sample being within a certain Ph so it can work correctly. As the RO water has no buffer system the reagents themselves can actually shift the Ph out of the correct range and effectively give phantom results.

If you mix a small amount RO water with phosphate free marine salt and then perform a test it should give you a more reliable result.

If there is a concern of stray nutrients getting through the RO unit a DI resin pod can be added after the RO membrane.