I meant RO water.This is tap water that I'm using and I haven't added any water conditioner yet.
I use a plenum as well and I have found that when I use root tabs the nitrates take a big jump and lasts for 2-3months (I assume this depends on the brand used). I switched to liquid ferts and nitrates have dropped off alot.Weren't root tabs meant to release their content over a long period of time, seems like 3 weeks isn't a long time?