Brother and Dad are right the cover wont keep the pond from freezing unless winter is very mild. I've used various deicers over the years and in Toronto Canada I have been fine with 300 watt versions and I really think that they are the best option. If a 1500 watt de-icer didn't make a hole in Zone 5a it was defective..(was it the green 1250 Watt stock tank heater by any chance? I had nothing but trouble with them getting warranty on 3 in 4 years until I got wise and stopped trying)

There are other do it yourself options some use light bulbs (if you can still find 100 Watt bulbs) and others use air stones though I have found the airstone systems to be a problem in really cold conditions..

All my fish survived last winter. I used both a powerful bubbler and a heater. No cover.

I use a stock tank heater, it crapped out last winter so I threw the de icer in and everything made it through the ice was a good 4" thick tho. had a good spell of -40 try a new de icer