Chris Hogg wrote:

It may help to keep the water clear if you add a handful or two of
ground or powdered limestone to the pond from time to time, to help
settle any bentonite in suspension, especially if the bentonite you're
proposing to use is from Wyoming. With some calcium dissolved in the
water from the limestone, the clay particles will clump together more
readily and settle quicker; the technical term is 'flocculation'.

Our water is so hard here, it might even have enough calcium in it
already, but this is good to know!

Another thought, some processed bentonites are 'soda-activated' which,
apart from making them much stickier, also makes them quite strongly
alkaline, rather like fresh cement. This will harm and probably kill
any fish or plants you introduce unless it's neutralised, and I'm not
sure how you'd do that after it's spread on the bottom of the pond, or
even before you spread it, come to that. Check that it's not
soda-activated before you buy.

The kind sold here is especially for ponds so it should be OK. But I
will file the info to be certain!