I've been weighing between digging a settle chamber out of liner or
using a 70g rubbermaid stock tank I have on hand. Today I'm leaning
towards the stock tank with a possible eye for upping it next season
or 2.

My ultimate pond goal is to have filter tanks that drain. Either via
pump or pulling a gate valve to a sump pit with sump pump.

This settle tank will be gravity fed and of course will be buried in
the ground.

But it's FLAT on the bottom. Does anyone have any clever ideas as to
how I can create the most maintenance free situation for this settle
chamber? How can I create a method to totally drain this tank to
remove settled solids?