disc element 18 within the tube 11 is indicative of the water flow rate through the pump for the particular valve 22 setting which may be indicated by providing range indications along the sides of the slot 46 in the pump body 1 moreover because the internal grooves 25 in the valve hub portion 36 communicate with a low pressure zone within the pump inlet 23 near the rotational axis of the impeller 8 as long as water is being circulated through the pump air will be drawn through air passage 47 in the knob 20 down through the axial opening in the shaft 7 and out through the bottom open end of the shaft 7 and then back through the internal grooves 25 for discharge through the flow passages 43 in the valve 22 adjacent the axial inner end of the hub portion 36 of shutter portion 21 to oxygenate the water being circulated through the pump it is also possible to change the amount of oxygen being mixed with the water by changing more or less the size of the air passage 47 through the knob 20 this can be accomplished in various ways for example by replacing the knob 20 with other knobs having different size air passages by furnishing the knob 20 with a control screw or by attaching to the knob 20 a small tube 31 furnished at one end with an interchangeable head 48 for obtaining different aspiration action from the foregoing it will now be apparent that the pump according to this invention has.