Since you are listed as being from Michigan, here's my 2 cents, I'm originally from Wi).
I have kept a 500 gal pool/pond upstairs in my house, a 1000 gal pool/pond in the basement and about 1000 gals in tanks throughout (all tropical tanks were individually insulated with styrofoam sheets).
In mid winter, Jan thru Mar, my energy bill, which includes water heating and pumpage. The fish portion of the bill ran between $300 to $400 per month.
I moved from that house, back in Sept, and to make the house ready for non-fish keeping residents, the entire house needed to be re mediated for for mold. Slight water damage repair was also needed.
My insight would be, if you are going to do an indoor pond, you need to have some sort of moisture removal system put in ahead of time. And also insulate the room, and the pond well before cranking it up. The important work done ahead of time, will save much in the end
I would also add that most natural colored fish will be hard to see, unless they are monsters, if the substrate is a natural color. I have found that using a grey or black liner, and a depth of over 2 feet, most fish will much of the time disappear.