Cold.
Room temperature is normally sufficient but in the summer months when temperatures exceed 23/24, try and keep the sun out the room of your tank or if its small enough, move the tank to a cooler area. In the winter it shouldn't really matter because the heating is normally on!
it wouldnt be "kinda chilly" without a heater as in our heated homes "cold" water unheated tanks are usually kept around sub-tropical temperatures anyway.
Technically they are sub tropical going on coldwater fish, they are find in coldwater temps as long as they don't get near or go down to freezing temps, room temperature (often around 17-24 degrees) is fine for these fish- i would avoid going into tropical temps (24 degrees+) though if you can unless the tank is very weel oxygenated, as oxygen disolves in water less the warmer the water is. If the tank gets big temp fluctuations during night and day (particularly during the winter time), i would advise maybe putting a heater in the tank to help keep the temp more stable .