Hi there. Can anyone help me. We had a large Koi pond with a wonderful collection of large fish but unfortunately a power cut whilst we were on holiday caused all filters etc to fail and sadly all the fish died except one. We were clearing out the pond about 6 months later and found a large dark brown fish, presumably a koi that was about 7" long. It had survived the power cut and also spent 6 months without any food or treated water and it was a complete shock to find it. Never seen it before. We have another very large pond for goldfish and thought he could take his chances and we moved the fish to the other pond. Never saw it again, only the 11 goldfish we put in there. This all happened about 5 years ago and now I am curious because we have over 300 goldfish some of which are extremely large and there are a couple of dark brown large goldfish size fish in the pond. In addition. We have one enormous half red and half brown fish. My question is this? Can Koi and goldfish inter breed because I can't explain the size of the goldfish any other way. Also if the koi was brown, could its progeny be different colours. We have some very large pale lemon goldfish. I am waiting with baited breath for someone to come back to me with any help. Thank you for reading my post.