Hi we had Koi for 12 years , we kept them in a 4 foot deep 6000 gallon raised pond , they used to feed out our hands , most were over 27 inches in length and were like pets , then 7 years ago we moved to Swansea in south Wales ,the fish were all bagged up in boxes by a proffessional Koi dealer , they were fine for about a year ,then during the winter i noticed a fish of about 7Lb in weight laying on the lawn with its head missing , so i checked the other fish and most of them were gone apart from my best fish , i knew then that they had been attacked by wild Mink ,there were fish scales every where under the caravan , so i doubled the netting over the pond ,and all was quiet for about 5 weeks then i noticed a fist sized hole in the netting ,my heart sunk i knew that my last fish had gone , apparently mine wasnt the only pond there was a few more cases reported in the paper ,so if you live near open ground beware ,mink are expert swimmers & climbers ,and are vicieous predators , back to keeping Koi ,never drain all your water down ,i used drain about 25% every 2 weeks , do not overfeed ,any floating food after about 5 mins scoop out with a net , and do not feed when the water temp drops below 50 F