I use a LMIII for auto water changes, and I love it!
I have it setup to exchange 1 GPD or 3.78 Liters (~5% / week of my total volume). I do not use the LM top off module at all, for ATO I use a Tunze Osmolator.
LMIII auto water change setup:
Pump B removes 1 GPD from the sump near the incoming tank overflow and pumps the waste water into a sink drain. Pump A adds saltwater from a 44G Brute container to the sump right before the return pump.
Since the LMIII breaks down the total volume specified into 125 increments over 24 hours and both LMIII pumps never run at the same time it works out really well. I've had this setup running for about 5 months now and the trace element parameters are rock solid (I still dose 2-part using another LMIII pair). Our tank is thriving and best of all regular maintenance chores are greatly reduced.
Cheers!
-TDF
