You can over water in hydro if you are feeding in intervals and leaving standing water in the bucket , even if the water was oxygenated it wont stay that way in the bucket for a prolonged period. Good Luck..
Ah, I used gravity return on my system like this. I'm guessing you used a return pump? Mine never got a chance to go stagnant when the pump was running, the buckets were always overflowing back into the rez. Never did like that two pump system for that reason.