Fog juice tends to leave a film, so I wouln’t run it over a pond stocked with live fish, unless you have a good filtering system.
If you do go the fog machine route, keep in mind that the fog from the machine will be warm, and will rise.
To keep the fog close to the ground, you could run the output over a chunk of dry ice (will it’s usefulness never end?). I recommend this over the dry ice in water method, as fog machine fog is longer lasting.