If you are interested in aeration you should research articles here about bottom diffusion aerators vs surface aerators and the pros and cons of each.
I can't help you with how you might use bentonite. Not many success stories with bentonite on this forum unless it is mixed with the clay ahead of time when working the layers of clay as part of pond construction. It might have helped if used at the time of creating the base of the dam but without a core trench you still may have issues.

I have had success with soil floc. I think it may take a re-application at times but your advantage is you know where the leak is.

I'm curious how a 'pond digger' can get any customers with such poor skills and limited know how. You would think they would understand the basics of building a dam? Maybe there were a few good references he used to sell others on?

I get so discouraged hearing these stories about people promising they can dig a pond and yet they know they only know how to dig a hole in the ground.

We need a 'do not call' list or a 'spam pond builder' list by region on this forum to save countless dollars and heartache for others.