They don't freeze... they slow down when the water gets colder.. they will hang out at the very bottom of the tank/pond under the ice build up on the top. They don't need to eat during these times either due to the fact that they are in a state of almost hybernation (not quite but almost). They also need to breath less which allows for them to last longer in this state without using all the Oxygen from the waterinstantthc;1938765; said:koi are very resilient fish, since they can be frozen over the winter. if u are going to do a kids pool i guess it's expendable. if so put a valve at the most bottom part of the pool. heat it and mold it if this is true. get at least 3x the filtration the pond is going to need. if 150g have a 450 gal/hr