In recalculating, the pond is probably closer to 1,000 gals, but it only has 4 small koi (7", 8", 10", & 15") and a few small goldies. In 5 years, I've only lost two fish, probably to a duck that was visiting the pond at the time. The floating pond deicer is 1,500 watts.
15" isn't small and they can get to 30+ inches; Look at the sticky at the top of the Pond Construction forum and note the weights and sizes and bioload koi can achieve; it's a bit eye-opening. Now, I'm not saying to re-home your fish (which should happen before you lose more in the future IF you don't compensate) but will suggest you start planning your LPS entry! LPS; larger pond syndrome--it's real, dude! What this means is that you're probably going to go large in the future, so why not do it before your fish start making your pond hard to manage? Just an idea. Otherwise, as they continue to grow, you're going to have water issues if you don't compensate with really efficient filtration. Consider a bog for this purpose. I know I'm not one to talk (too many/too large fish!) but a small pond is a LOT harder to keep in great shape than a larger one. Another reason to consider 'movin' on up'!!
But trying to heat a pond, even a smaller one like yours, is going to be hard; if you get a decent sized heater, it's going to cost $$$ and you might get the temp high enough to be of value. Especially exposed to the elements, you're now fighting Mother Nature and well, She's been around a lot longer than you, so...my money is on THAT Mistress, ya know? A better idea to keep some heat in is to erect a winter tent to help keep in the earth's natural heat and stop evaporation. Ask me how I know, this...
Your fish should be okay as long as the ice doesn't get as low as the depth of your pond. Anything above ground is much more likely to have this happen and the deeper pond, the better winter (and summer) protection!
Okay, so, get on that idea horsey and 'Let's Ride!'
Ah, soooooooooooooo nice to be able to say that again! Look out 'eyes', I know where ye be a-hidin' now, got my trowel and mortar lined up right next to a bunch o' bricks!!!