Reservoir Nutrient Stratification

My plants in a Hydroton Ebb system are showing excess nutrient stress. I dipped my tester in the Res. and it was very low. I stirred up the solution and it read normal. If the nutes are settling toward the bottom of the the res. and the pump draws from the bottom then the plant get a shot of concentrated solution initially before things circulate.

I flood for 10 min. it takes 7 min to flood then I just want to mix the solution by letting it circulate. I am running a dual air pump with air stones but it doesn't seem to circulate the water much.

Any tips?