I was thinking something similar but instead of having the system pump water out to be dumped into the drain, just put the 50G as part of the whole system... Have the water trickle through the 50 G at a slow rate then dump the whole 50 G every month or 2 to be replaced with fresh. Then the overall system volume is the DT+sump+ the 50G and you're basically doing 50G water changes every month... maybe?
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