Well folks so far so good with the fish in the unheated garage. I bought small aquarium heaters and alternating them with higher watt pond heaters as needed. Trying to keep 38-40F.
Problem is the ammonia levels are high. Have to change out water every 2 weeks to keep it down. This is not the best way to overwinter fish.
We continue with our routine water changes at specific times during the winter and change 40% of our ponds water every maintenance weekend (flexible either saturday or or sunday) it doesnt matter to do it on a specific day .
My Canadian friend in her latest email to me has changed things and now runs two indoor Vats the filters from her can be detached from one another and both still mature run the vats throughout the winter with the addition of extra piping
She does a regular water change of about 30% per fortnight , her koi are kept at 11c in her husbands workshop throughout the winter only returning them to the pond in late spring.
She employs sacks of zeolight leach out and store any ammonia present .
The upshot of all this is she has gone from loosing koi to intense cold to not loosing a single one so if done correctly there's alott to be said about overwintering koi indoors ,
Dave