Ok....sorry and I'll put my fur back down too.

It HAS to be one of those three things. Your light is good from your photo and your room temps are good as well. You're doing everything right from what I can see except for your low ppms.....and the rockwool. I've heard of issues with it regarding both holding too much water (overwatering) and having a low ph but don't use it myself so have no real personal experience with it.

Your ppms may be rising but not necessarily for the reason you mentioned. When was the last time you changed the rez completely? I would make that a weekly thing if you can....complete res change.

When the plants eat they pick and choose what they want from the nutes available. They may snarf up TONS of nitrogen and leave the potassium behind....but it won't show in your numbers which simply reflect the TOTAL of ppms in your water....not the ratios of each nute. Do you see what I mean? Thats one of the reasons a res change is so helpful.....to restore the correct balance of nutes.

Checking ph with a liquid test is fine....but DONT use any ph adjusters for aquariums...they are way too high in salts for your plants. (just fyi)
Weedhound Reviewed by Weedhound on . Can't figure out what's wrong... 3 weeks from bagseed, 1" rockwool cubes (pH buffered) and hydroton, 8" net pots in five gallon buckets with bottoms cut out. Flood & Drain / Ebb & Flow, 40 gallon reservoir. (res temp 71 F) RH = 40% Growroom temp = 75-78 F 250watt HPS light, with two 48" Floro tubes as side lighting, and a CFL for good measure. Rating: 5