Will bentonite harm fish in a pond?

yes it does and no it doesn't.

Bentonite itself is not particularly "toxic" but the very small particles released into or maintained in the water column during installation WILL clog the gills of fish. How serious this is to the fish depends on the amount of it of course but can easily lead to asphyxiation if in excess.

So fish should never be present during installation and some time after until the material has bedded and loose particles flushed out. After that it is perfectly safe and some would even say beneficial.

PvP

There ar e two forms of bentonite: Calcium bentonite and Sodium bentonite. For ponds you need the former, calcium bentonite. It should be given time to settle where it will form a clay layer often used to stop leaking of ponds into the sub soil. For ponds that won't clear seeding with gypsum often helps to flocculate the suspended particles helping the filter to trap them.