Not sure on the ppm, I couldn't afford the $120 ph/ppm meter and opted for the $60 ph only meter instead. I changed the res last Thursday (4 days ago) and added 1/5 then (45 ml in 15 gallons). Do I have to change the whole res again or can I just add nutes to it? As I ask that, I am almost sure the answer will have to do with the ppm. Of course....

Oh, and they are probably around 4 inches tall and have been growing a new set of leaves every 4 days or so.