My koi get them without having to feed them as they fall into my pond. Redworms are found in the rocky bottom of my pond which help to clean but are also a "search&feed" game for my koi.
In my area we are in a pesticide free-zone and my garden is 100% organic. The only ferts my garden get is that from the waste water of the pond. Koi of course are a little more resilient than tropicals and as the others have stated it is risky. If you wish to feed them earth worms, redworms or grubs go to a local fish rod&reel bait shop and get farmed worms. All fish love them for the most part and it's a good thing to give live foods but only if from a clean source.