I'm with you 100% on the reasoning, and use your waterpump design with slight modifications. My only difference, and I haven't had a mold issue is, I put CSM + B and N & K in the main container. Manually dose P.
In the future I might by a pair of peristaltics (one for each tank) and dose Trace that way and put NPK in the container/water pump.
Three Tanks...Eheim 2128 & XP3-90G, Eheim 2128-65G, Eheim 2232-25G.... Tek 4x54 watt T5-90G, Aqualight 96watt PC 65G low tech, 65 watt Aqualight-25G.... Hydors-90G & 65G ... Flourite in 90, 65, & 25 Gallons, .... Auto Water Change/Auto dosing on 90 & 65 gallon..... AGA member......