200 gallons really worries me - that's just not a lot of water. And 18" isn't deep enough, either. 2 feet is the minimum recommendation in our zone 5B. I would plan to bring those fish inside to be on the safe side. I mean, if we have a mild winter they might be fine - but by the time you realize it's NOT it'll be too late.
It's the WATER temperature you're concerned with - not the air temperature. Your pond will cool slowly - water doesn't have dramatic drops and rises in temperature like the air does.Our temp went to a low 46 degrees last night, but it's a beautiful 72 degrees now!