How big is the fish tank?
The rule of thumb is to pump at least the volume of your fish tank each hour.
To figure out big the pump needs to be, you want to find a listing that will show how many gallons per minute or per hour the pump will move at different heights. A pump listed as a 300 gph pump will probably only pump 300 gallons per hour at 0 head (that means as soon as you try to pump the water up a little bit, it pumps less water.) So, You will probably need to make sure your pump can move the volume of your fish tank at some height between the height of your grow beds and the height of your towers.
Will you be pumping constantly or will you be using a timer? If pumping constantly you will want a pump that will move at least (if not more) the volume of your fish tank at at least that height between the grow beds and towers. If you will be using a timer and say pumping only 15 minutes each hour, then you need a pump that will move at least 4 times the volume of your fish tank at that height in an hour.