Be carefull. Simply reducing flow by restriction will increase stress on a pump. A process similar to dead heading. Instead put a diverter or Y fitting in the outgoing flow. Add a valve for adjustment on the line to the beds. The other line returns to FT to recirculate, and aeriate the water. When valve is completely closed, the water simply circulates in a small loop. When open, very little water by comparison comes out the disconnected end. Use extra pipe to route your non FT end back towards the tank to increase hydrostatic pressure/resistance on it, but also to have better control of extra water. No sense having it fly loose and get water on everything. PVC 90's and 45's work good for this.