Yellowing leaves, please help!

I have plants that are showing a really really light green/yellow on almost all the leaves. Ive narrowed it down to either overwatering or lack of nutes. Ive been using a 15-10-15 drop mix that says "use everytime you water" and it was doing ok, but now the leaves arent very green. I water every 3-4 days, 400w hps, 81 deg room temp, 10 inch pots. I have two in hydro and they are a dark green. Someone told me i can use the water from the hyrdo res to get the right nutes into the plants. I just flushed them and was gonna give that a shot. Didnt know if the pots were to small either, most people ive seen use this size so i dont think its a problem, but with the flushing i would assume i def need to get some nutes back into the soil. Any suggestions anyone? Please help!! Thanks guys!

How old are they? Do you have any pictures?

Do you have a way to check PH? If it's not nute burn, my guess would be maybe low PH. Nutes will push the PH way down.

Good Luck,

Sheepdog

Ill try and get some pictures up tonight so you can see.ive been checking the ph levels of the water and they have been at about 6. Ill get pictues up asap so that you can get a better grasp. Thanks!