Catfish are bottom feeders and will most likely keep the bottom stuff stirred up. They also get big and will eat anything they can fit in their mouth. I'd take them out, and depending on your preference, fry them up for dinner or let them go back where they came from. I would do the latter. You will probably have a heck of a time catching them.

To deal with this nightmare you will have to remove your koi and drain down the pool to as low as level as is possible to spot the tops of the catfish or to see their movement in the shallow water. You will need two fish nets and it will take a couple of you to corral them.
I would not reintroduce them back into any lake apart from the place they came from. Failing that cook them and feed them to cats unless you like the taste of catfish. They also will be eaten by any wild animals such as raccoons or possibly possums if such occur in your area. And finally bury them after cutting off their heads as fertilizer. These kinds of fish can survive in a variety of habitats and even if there are no fish to eat, they will eat decomposing plants. They are one tough fish.

We had a catfish named "Wayne" for years in our Koi pond until he got sucked up in the new pond bottom drain - long sad story and the BD's dome cover was off.

He was no trouble- but he didn't have a mate either. The Koi didn't mind him and he was very tame.

If the catfish are very small then you can use a minnow trap or make a fish trap. Always ideas on the internet.

Regards,
Lisa

Koi and catfish can live in harmony for a while, but as the catfish grow they may damage or kill the koi. I have seen it happen. A potentially more serious problem right now though....those wild-caught catfish almost certainly have some sort of parasites...nearly all fish in the wild do. And now your koi have been exposed as well. It is never a good idea to put wild fish in with koi.

I have albino catfish in my pond with my koi. Everyone is about the same size and last summer I noticed while handfeeding my koi that the catfish wanted to handfed also. It was kind of startling seeing this wide gaping mouth come at you but I got used it. Everyone gets along fine with no problems. Of course, I got my catfish at pet store.

Are we all talking about the same kind of catfish. I'm speaking to channel or bullhead catfish. The kind people use hook and line to catch and eat. If small they will be OK but they do grow larger. Parasite and disease are another issue already brought up and since there has already been exposure not much you can do about that now. I'm glad some folks have enjoyed their catfish and found them tame coming up to be hand fed. That is good to know so maybe they can stay and enjoy their life in your pond. If you don't want a bunch of baby fish it might be a good fish to have around provided you haven't experienced any trouble. Maybe let them stick around for a while to see how it goes.

I have 5 bullhead catfish in my pond. I have raised them from babies. They have been in the same pond as goldfish but its not recommended as they will eat anything that fits into their mouth once they are bigger. They are hand fed and love being pet. In the winter I drain my pond and I take them inside for the winter. They are really cool fish to watch. I am also getting an albino catfish soon!