Question
Hope you can offer some advise, I'm getting very frustrated. I have a small pond ( 5 ft x 5 ft) and 4 ft deep. I keep goldfish and Koi, with the addition of turtles through the good weather.  Have had it for 6 years now and only one successful winter. I cover it with screening on a wooden frame to keep leaves out. I clean out bottom debris before winter. I change feed to a spring and fall formula before stopping at 50o. I live in Toronto, Canada. One year I tried a small bubbler dangled at about 6", only to find in the spring that the fish had been circulated up onto the ice and died. Then last year I changed to a pond heater and all the fish died to the bottom.

This year, the ice has just gone and sure enough there were my Koi floating at the top, but the goldfish appear to be healthy.

I sure have lost some beautiful fish over the years and would appreciate any tips.

Answer
Hello Bruce!
I am sorry, but the news I have to impart on you is not that great:
The pond is much too small for Koi! Koi need at least 500 gallons of water for EACH fish. In the winter (and in your climate) need a depth of at least 2 1/2 to 3 feet and plenty of oxygen to survive.
You will do much better over-wintering the fish is to stock the pond with Goldfish, maybe including some of the more colorful ones like shubunkins.