Depends on what you like. Mosquitofish, White Clouds, some Gobies, Neon Tetras, XRay Tetras, lots of Catfish, Danio Rerios, lots of Botias, etc will all do fine in Temperate conditions depending on the PH and circulation. Even Guppies will do ok in the low 70s if there isn't too much Endler in them (feeder Guppies).
They are a schooling/shoaling fish so the more the merrier. What’s the footprint on a 40 breeder again? My three and the ropefish will often be together in a pile. They are almost never alone and when my male died, the two left were obviously upset and missing him. They didn’t leave each others side for a few weeks after.