as a long time aquarium hobbyist…
cool is the key (slows metabolism, slows bacterial growth and increases oxygen carrying capacity of the water). and having your makeup water sit out for a day or two largely eliminates toxic chlorine (chloramines in some water supplies are more stubborn so using an aquarium chlorine/chloramine product may be helpful) and ensures that the temperature is the same (so as not to shock fish with a rapid temp change). an aerator or pump to provide circulation will help but not absolutely necessary if water is cool (gas exchange occurs anywhere air and water meet, be that in a bubble or at the surface and even a ripple helps). the enemy is water pollution. and the solution to pollution is dilution. the more water volume the better.
if you choose to keep inside more frequent water changes and a filtration system would be in order. lest anyone was thinking about doing it-feeding minnows (like you would an aquarium fish) is only going to worsen the pollution issue. and it takes longer than you would think for them to starve. if you have minnows starving you should be thinking about ways to get out fishing more, not scheming about how to keep them alive longer;)