ya, I'd never thought of them to be fish-eaters, but was then surprised to read that.
but yeah, I doubt they'd be detrimental, they are mainly plant-eaters.

I looked around and saw no evidence of any burrowing.
they also make a push-up den, more similar to a beaver's.
I was thinking that maybe having a resident muskrat might help keep other things from coming in.
his lake is very close to a main river, and I know there are otters in the river, so I'd expect visits from them if there are fish.
the lake was done properly, it's aerated, it has a big waterfall also that continuously flows, and is 18 ft deep at it's deepest.
I don't think he's too concerned about losing the odd fish, it's kind of expected.