I would use those windows to your advantage; free light, as well as ventilation. A big problem with fishrooms that is often overlooked is ventilating out the humidity. I imagine you have colder winters in New York; humidity will cause condensation, and eventually mold, which can cause health problems.
Insulate! Betta do like it cooler than the angels I breed, my finished basement will get into the low 60’sF in the dead of winter, when the heat is turned down at night. Along with insulation should come a vapor barrier, once again to contain humidity.
Plan ahead for electrical & water usage, working with this after the room is up & running is a real pain, it’s easy in an empty room. Whatever you think you will need for electric, double it, as doing an electrical upgrade is either a lot of work if you do it yourself, or costly if you have someone else do it. The idea is to make the room flexible as well as efficient. You are into betas now, down the road you may get into discus, livebearers, who knows. If I weren’t doing angels I would do L number plecs, which would work just as well in my setup.
The stairs going downstairs need a little visqueen curtain at the least, or a door if you want to get into some carpentry work, another humidity/heat conserving thing.
Any questions feel free to pm me, or post them on the board for the knowledge of others. I have seen many fishrooms, everything from a half dozen tanks in an oversize closet to 300 tank setups taking up a good size building.
