Koi can remain outdoors so long as the pond does not freeze too deep. A 6-8 inch thick ice cover over a 3-4 foot deep pond is acceptable so long as a substantial area of surface water is open at all times to allow for gas exchange.oi can remain outdoors so long as the pond does not freeze too deep. A 6-8 inch thick ice cover over a 3-4 foot deep pond is acceptable so long as a substantial area of surface water is open at all times to allow for gas exchange.