Hi
It depends on what kind of winter you get. Im from the uk so you will have to say if the temp gets a lot of the time in the sub zero range.
At a foot and a half in cold climates where the likelihood of a freeze will occur i would say bring them in as i dont think the pond is deep enough to allow the fish to escape any freezing water.
For most frosts 3 feet deep is required as a minimum.
if you can try and get a 60 gallon tank or rubbermaid with lots of filtration you wont need a heater if its in a indoor room and your fish will be ok to back out in the late spring again.