That controller is specifically designed as a top off controller only. It has 2 major drawbacks for your purpose. 1.)it has an audible alert when pump is active. Which for you would be all the time. This can be disabled by removing the speaker from the circuit board so not a real big deal. 2.) It has a "float fail-safe" feature which only allows the pump to run for 3min 45sec. It can be extended to only 14min. Which means, in your case, it will shut your main return pump off every 14 min. and will need to be reset.
This controller will not work for you. Find a simpler one or build your own. They are super simple.
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