We have a goldfish who has been on his own in his tank for a few years, we tried to introduce a small koi carp that we were given but he was very agressive towards it and we had to remove it. Now it's bigger we tried to reintroduce it but he's attacking it again, is there anything we can do? The tank is 24" x 12" x12" so I think it should be big enough for just the two of them.

Thanks for the reply. I know it's not a huge tank, but the goldfish is several years old & only about 4 1/2" and the koi is over a year old and only about 3" so I think they should have enough room for now, if I'm wrong please tell me, I'm on the point of separating them again anyway, the little lad looks really miserable!

I agree with fantailer :/ That tank is pretty ridiculous even for a goldfish. Did you know that one goldfish needs a 20 gallon tank to thrive and grow as big as it's supposed to? And to even attempt keeping a koi with it is... please give that koi to someone with a pond! You are wrong if you think you can keep an adult goldfish and an adult KOI CARP in such a tin can of a tank :/
Even if they have "enough room for now" doesn't mean they'll be healthy! They still produce waste, which is the main reason they need a certain amount of water not to poison themselves. If you want to keep them together so bad, build a good and proper pond! But since these two fish don't get along, I'd say it's pretty useless to force them to be in the same tank/pond :/

Final answer: Give the koi to the pet shop or someone you know that has a pond, and keep the goldfish. If you'd be extra nice to your goldie, you'd get him a bigger tank