Whats up...

First, a PPM meter is very important. The yellowing then back to green sounds like the plant is running out of nutes then greens when you finally give it more. With a PPM meter you can keep it dialed in on the healthy amount. If you had pics it would be easier to identify if there is a problem.

A PH pen also helps to keep the ideal PH of 5.6 - 6.0 if I remember correctly. From my experience hydro is much more critical for the ideal range of PH compared to soil so keeping it at around 5.8 is best.

Hope I could help! :stoned: