A 6" catfish is going to have a hard time eating a 5-7" Koi. Chase, sure. Bite fins, sure. But eat would be difficult. Just watch them. The Koi may chase the catfish, or catfish chase the Koi, or they may just leave every one alone. Well feed fish are less aggressive imo.

I don't know your climate or kind of catfish but it could be 12" by the end of summer, so could the Koi. That size pond, unless you have serious filtering you'll probably have to be looking for several new homes.

Thanks so much for your helpful insights on my issue! I live in Southern California, so the temperature is pretty much the same year-round. I'm not sure what kind of catfish I have.... But it looks in my opinion like a flathead. Anything particularly dangerous for this species? Also.... The pond is quite well established, it's already been 3 years old since I first created it. I have added about 5 inches of dirt to the bottom (big mistake) where waterlilies now ravage the bottom of the pond. This makes the water quite murky... Which I heard catfish like. This also makes it quite hard to see the bottom of the pond completely. I don't really know what the catfish is doing, since it dwells at the bottom, but as far as I see, the koi and catfish aren't communicating. But I did notice that now the koi tend to stay more towards the surface of the water. They have no visible scratches or anything so I'm not particularly concerned. Also, when I feed, the catfish never comes up.... I'm just afraid the catfish will grow larger faster than the koi and in no time, start eating them. If I do choose to take the catfish out, will it be okay in a little 20 gallon tank? THANKS

-Cat

No way to tell if 20 gals is enough without knowing what kind of catfish. I mean flatheads get over 100 lbs, so in that case 20 gals would be kind of tight. For sure don't release it into the wild.

Catfish will come to the surface for food but may have to be trained, or learn on it's own.

They can grow fast, but can be stunted too.

Catfish don't need mud. Ponds with catfish get muddy because they do look in the mud for food so a lot of people think they must like or need it. They can find food on bare floors, on the surface. Catfish farms feed fish on the surface.

I would say all fish, Koi included, would prefer muddy/green water. Protects them from the sun and predators. We prefer to see them. Such is the life of fish.

I have 18 Koi and two Channel cats. One has been in the pond for 5-6 years and about 18''-20'' now. He has never shown any interest in the other fish. It took about 2 years befor he started feeding at the surface. Thy are a welcome addition to my pond and very well behaved.

Same results as Mike56.... channel cat has been in my pond for 4 years and would estimate to be 4-5 lbs and has not bothered my goldfish and Golden Orfe what so ever.