This is just a guess on my part as well. (We need the hydro folks here to answer this question.
I have my tomatoes in rockwool surrounded by clay pellets. I have about 1" of the bottom of the rockwool sitting in the water as the roots are yet to protrude through the rockwool and dangle into the water.
I'm thinking what you want to do is have the water barely lapping the bottom of the rock wool and not soaking in it? I'm thinking you just need sufficient moisture to keep from complete dehydration and the plant WILL seek out the water and grow the roots down.
Also, what are the specs for this air pump that is supposed to be good for 80 gallons? For my AACT, I started out with 1000 -- totally inadequate -- then upped to 2000 and now, I'm using the 3000. Almost no such thing as too much air pressure it seems.... The stone clogs very easily especially in presence of nutrients. I have to soak in peroxide to clean every so often (can't remember exact span of time before needing to clean).