Pest control poison harmful to pond fish?

Dust or spray would be worse than something the mice were supposed to ingest. If they just get it on their fur, then are supposed to absorb it that way or eat it while grooming, there is a chance they could make it to your pond before the stuff take effect, and some of the compound would naturally come off their bodies and into your water. As the previous answerer stated, how bad this would be for your fish would depend on the type of poison, the amount, and the volume of your pond (a few grains in a few thousand gallons would probably be survivable, but that same amount in a few hundred gallons might not be....).

If the poison is a pellet or some other kind of bait that the mice need to ingest before it is effective, it is probably going to be OK, unless one were to carry it over and drop it in your pond (possible, but unlikely). Even if one ingested the poison, then came near your pond and dropped dead into it, there would be a greater danger to your fish from the ammonia spike caused by a decomposing body, than there would be from the poison at that point. We use solid pellets for pest control around our pond on a fairly regular basis (the type that inhibits blood clotting and results in the mouse basically hemorrhaging internally - I think dehydration is a side-effect, too) and we have never lost any fish due to cross-contamination.

Hope this helps.

Not good. The 'critters' will almost certainly contaminate your pond water! A lot would depend of course on the water capacity of your pond and if it is filtered.

The 'Pest Control' people I am sure will give you the very best advice. Seek their help and advice now.