Go as deep as you can. "Too deep" is a function of safety (aforementioned slope) and pond size, subsurface soil/rock types, and your wallet.
Does this apply only under the premise that the pond is aeriated?
From what i understand a deep unaeriated pond might have a zone with low DO below the thermocline,
so when the thermocline is disturbed - for example in a big rain event or sudden cool down - the water is all mixed and
therefore the DO in the upper layers is lowered possibly resulting in fish kill. Is that correct?
