My Boyfriend bought a home with a Koi pond Last July, Besides alot of alge the fish thrived. Most were in thier teens, some smaller ones. We were told that during winter the fish would hibernate and not to worry. We did NOT use a heater, we DID keep the pump running all but 3 days. We live in Denver,CO. and when the pond thawed we found dead fish, 5 so far out of 8. What did we do wrong? What are the best steps to take now to keep our remaining Koi alive and to make the pond a better environment. We still may get some snow in April also. Can anybody help us we are very sad about this whole thing, but want to bring the pond back to life. Thanks for any replies.