2 dos units one for two part the other for nutrients.If you already have an apex, get a trident. It measures alk, ca, and MG as many times a day as you want. When your alk is stable, it provides a great heartbeat for the tank. I can look at the ups and downs and compare to previous days, and if corals are comsuming more or less alk based on any changes I made. Pretty awesome and I'd never run a tank without a trident now.