Yes you can overwater hydro - simply meaning you deny the roots oxygen. Not sure how your system works, i like to have an air stone in my resivor to make sure plenty of oxygen gets to my roots. then i flow/flood multiple times a day with that oxygenated water. i'm not sure how this works with nft, i'm not sure how yours is setup but i would imagine the cubes are just soaking up a thin layer of water, maybe a flow system would be good for you. I hear alot based on feed schdules, and feeding like 10 times a day with a flow system in veg has brought my outstanding result's and i'd like to blame it on the fact that my girls got fresh oxygen all that time.