By law, you're supposed to contact KDFWR before you reclaim a pond, and the only approved way to kill off the fish is with rotenone. Maybe that's the chemical he's talking about. If so, the FDA hasn't approved the use of fish killed that way for food, but I know a lot of people say those fish are fine. It kills them instantly, so they ingest very little, and it's not very toxic to humans. And the pond can be swam in three days later. A friend of mine had his pond first stocked back in the 80's and it was full of catfish prior. They killed them all off with rotenone and gave them away by the bucketful. Here's a fact sheet on rotenone:

https://srac.tamu.edu/index.cfm/even...factsheet/219/