Can you keep a birdbath heater in it over the winter? That would keep it warm enough to keep from freezing. However, if you have a power failure during a really cold stretch, it could be a problem. I have a small preform in the ground, and use a birdbath heater every winter, and the frogs survive.
(Your's is in the ground, right?)
If you don't put a heater in it, I'm not sure your fish would survive.

35 gallon is pretty small for trying to overwinter fish. One good cold snap, and you might have fish-cubes.

For the cost of running a heater all winter long, you might be better to bring the fish in for the winter.

I don't bring mine in, but my pond is 30" deep, and about 800-900 gallons.

It would be best to bring them inside.
You can put a heater in the unit and it does work to keep it from freezing but it won't heat the water to summer time water temps. You can use one of the titanium aquarium heaters, don't use the glass ones. If they get pulled out of the water the slightest they break in the cold air.
I overwintered my first whiskey barrel water garden that way one winter. I didn't have fish in it but the aquarium heater did work to keep it from freezing. It never did have even so much as a thin sheet of ice on the surface.

thanks for the feedback folks. i asked a guy who runs a local pet store and he says i can cover it with plexi-glass with a 2 inch hole in it and he also recomended the birdbath heater. i am still pretty sketched about leaving them in there over the winter, but this is why i plan to thoroughly investigate all of my options before it gets here.
again, i really appreciate your replies.

You should tell the guy at the local pet store that his advice is utter nonsense! The bird bath heater would run continuously and cost you too much money over the winter.

It seems that 99% percent of pet store and garden center employees, know nothing about ponds, fish or how to look after them, but will make up and freely give out stupid advice, to all who will listen to them!
"Horton"

sortagreenthumb

I'm in Bot. Cty in swva - the most ice I've seen here is about four inches. You don't need a heater.
I simply use an inexpensive aquarium air pump and a bubbler.

fred