there isnt a need to winter fish indoors. there are just some things that need to be addressed when building your pond. if you live in a climate with harsh winters, you just need to make your pond deep enough so it doesnt freeze solid in the winters. a pond can ice over, as long as there is a hole in the ice for oxygen exchange. they make small heaters for this which keep just a hole about the size of a basket ball oopen in the ice. koin and goldfish will go into a hibernation type of mode when the water temperature drop to around 40 deg f. stop feeding when the water temp hits around 45 and turn off you pumps. this will allow the deeper parts of your ponds retain heat, and the lawer of ice also acts as insulation.
Can you get your hands on a 30 to 55 gallon plastic barrel / same as the ones you see used for a trash can. Cut it some where around the middle or closer to the top depending on how much space you are needing for your filter and pump.
something like this
Use a sawzall or hand saw to cut it / with a helper we have used a skil saw to cut them.
Cut it in the correct spot and you could use the other half for the lid to keep stuff from falling in filter box.
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