Automatic waterchange with top off design - please gimme some feedback

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Seen a really simple automatic water change system (believe was on RC) that was just dual matched dosing pumps running at the same time, one to pull from a reservior, other from the sump. Pumps were on a timer with on times throughout the day to change about 15 litres.

No high flow pumps, no float switches or water level sensors to mess up. Being matched wouldn't confuse a ATO.