I am using air pumps rated to 800L per hour, each ...

Ahhh I have around 30 buckets and Its I go on how much each bucklet is getting and I have found that it only really needs around 2-3 litres of air depending on your pot size. I have 30L pots so I usually use two air stones in a bucket of that size. though I also have 10 L dwc buckets that I made myself and I found that they take alot less. Though there is no such thing as too much oxygen in the root zone the more air in there the better and more surface activity will encourage more root growth from the sides of the pot which in turn equals bigger root system. More is always better than less in this question. 400/6 works our at around 66 litres of air per minuet now if that is spread over 4 buckets then thats way more than enough oxygen. if you have more than that I can only assume you have so much air movement in there you are actully evaporating alot of the feed into mist which is escaping from different places and this could cause her to not be getting enough. I can tell you now if the root system is not getting enough oxygen the leaves will turn brown and start to die quite rapidly. so its easy to notice if you look at my MBK diary the air stone in that recently failed without me relising and it began to die lucky i noticed in time to save her and she still going but yeah its something you need to keep an eye on cause nutrients can block air stones over time too so quite a few variables to check in our beloved dwc systems :) Best of luck man and happy growing :)