Growing up, we used to keep leftover minnows in the aquarium in my father's house. I don't remember doing anything special, standard aquarium with aerator and filter. When using tap water, you need to let the water stand in the aquarium for a couple of days to let the chlorine evaporate out before putting any fish in. We just let the water be at room temperature, which fluctuated between 65 - 75 depending on the season. It may be that the only minnows we used were shiners and that other minnows may be more sensitive to temperatures, but we never really had any fallout. Kept them just like any other freshwater aquarium fish.