It would be just fine. My grandfather moves his goldfish and koi into an aquarium in his garage each winter in upstate NY and I don't think he's lost a one - I would most definitely include a filter though. I hibernate some of my more sensitive turtle species in my attached garage each year since we live in a pretty variable climate out here on the Plains.
~Mike

Overwintered my fish in garage for two years and my climate is way colder than yours. First year heated garage but too expensive last year used a 1500W stock tank heater also too expensive so are coming back in the house. I ran into a problem last year as hadn't put the stock tank heater in and went out to the garage to find the pump spray thing had frozen up and only about a foot of water/ice left in the pond hence heater. Fish were swimming under the ice all okay. I used a 55 gal drum upflow filter full of spring flo and matala mats on the bottom that discharged through a spray bar into the pond. This added some air and filtered some of the poo out--not much biological when water temp is just above freezing. In the spring it worked great as by the time the pond was warm enough to put the fish back out the filter/water in their winter tank was cycled. When I move the fish in or out I pump the water from their pond or tank at the same time as I move fish. That way it jumps starts the pond in the spring and in the fall have cycled water/same temp when moving indoors. If you are using a submersible pump an Adam's filter would probably be sufficient for the winter as once water temps below 55F don't feed fish. This year I'm putting the fish back in the house and running a big Fluval cannister filter. My winter tank is a liner tank/ 2 x 4/plywood/insulation about 200 gals. Looks like a hot tub in my basement. Fish will be coming in this week as temps are dropping quickly. Sorry for long post. Good luck with your indoor setup. Patti

Thanks so much. I actually left my fish out for the last two winters and they were all fine. Last year, the bubbler top fell under water and the surface completely froze. It took me a while to get a hole melted again. I thought it might be easier to just bring them in this year and my main concern is whether 150 gallons will be sufficient water for 25 fancy goldfish or whether I should spring for the 300 gallon which will take up a lot of space in the garage, but would be feasible.

Deb, when I first read your post I wondered if you had to bring your fish in because your pond was too small and likely to freeze solid.

I don't know how large your pond is, but I can tell you that I had a difficult time catching 5 goldfish in my 1000 gallon pond. It doesn't take them long to figure out what is going on.

For me, it's much easier to maintain a hole in the pond ice during the winter then to catch all the fish and maintain a stock tank. Many of my fish have been in the pond for over a decade, I'd be reluctant to change their routine.

My pond is only 18 inches deep and 500 gallons. Last year the ice became pretty thick and I was afraid it was frozen solid, but I couldn't tell for sure. I worried all my fish would be dead, but when spring came and the pond thawed, they were all fine. Knowing the pond size, do you still think leaving them out is my best bet? Thanks!

I can only speak with confidence about the experiences of my own pond. My pond is 1,000 gallons and is 3 feet at the deepest part. I've never had more then 5-6 inches of ice in over a decade. I've used a variety of devices to maintain a hole in pond ice and they all worked relatively well. I currently use an aquarium air pump because it's the most economical method.

I've seen some preformed plastic ponds freeze solid in NE PA. All of the ponds that froze were above ground. An in ground pond will have warmer water at the bottom and is less likely to freeze solid.

I can't tell you that your pond will not freeze solid, but I do know people who have maintained in ground ponds smaller then yours in zone 5 without major icing.

If I had a 500 gallon pond that was in ground, I would leave the fish in the pond a run a large aquarium air pump with large air stones. I would also invest in a stock tank heater (1500 watt) as a backup. I had a two feet of snow turn to ice two years ago. The stock tank heater melted a hole down to my pond water in about 3 hours.

Deb your zone is pretty warm and if you left fish in for two years and all survived having them indoors for winter is more work and in the garage you won't see them as much anyways. There is a submersible Thermopond deicer that only uses 300Watts and another one--same brand I think that only uses 100 Watts. Like I said I used 1500W last year and power bills were high but had to run it 24/7 for 5 mths. Noticed you said you had fancy goldfish. Do you mean fantails? The fancy goldfish don't do well in colder temps. If your goldfish are wakins, comets, shubunkins they should be fine with bubbler and back up heater. Just make sure your heater is in the pond before you get ice. I think you can get some kind of timer thing that turns the heater on/off at certain water temps--saves on power bills. Good luck!

Jalal,

I'm not sure fantails are any different then comets when it comes to cold temperatures. I have four fantails that have survived ten winters in my pond.

Most of my fish are fantails, including three black moors and an oranda. They were all outside the past two winters. I guess I'll try the stock tank heater. I had a Thermopond deicer and it didn't seem to do anything. It became completely iced over and the only thing keeping a hole open was the pump bubbling. Thanks for the help!