You can not keep the temperature of your bait-fish-tank much higher than the water temperature of the body of water you are going to fish. If the the temperature difference is to much the fish will die very fast on your hook...in minutes or even in seconds,depending on water temperature differences.
If I remember right, the lowest temperature to keep the bio-bacteria alive is around 40 degrees.
The metabolisms of fish slows down the colder the water is and comes to a standstill below 50 degrees or lower, depending of species. So do not overfeed the fish. The less they eat, the less bio-bacteria is needed.
The colder the water the more dissolved oxygen it can hold.
The bait fish I am keeping are quite resilient and can handle some really traumatic shocks. But, that is actually a moot point here because the only time that the water temp of the tank and the water temp of the river where they are going to be used as bait in will differ dramatically will be in winter when I am not going to be fishing. During the spring, summer and fall, the temperature of the tank and the river water temp will be nearly identical.
The fish actually go nearly fully dormant (regarding eating) is around 40°F. But, that is about the same temperature where the Nitrosomonas bacteria die.
Nitrosomas bacteria thrive between roughly 75°F and 85°F.
Therefore, my goal is to maintain the wintertime temperature of the tank system somewhere between 55°F - 65°F during the winter, at least not less than 50°F.
Within this temperature range, the fishes biological activity will not be near zero, but still quite reduced and the Nitrosomonas bacteria will not die, although their growth rate will be reduced by 50-60%. Somewhere in this temperature range there will be that perfect balance and I can experiment over time to adjust my system to regulate the temperature to the optimum level. When spring comes, the heating system would be disabled and the ambient air temp would control the temp of the pond water. The population of the Nitrosomonas bacteria will be alive and ready to take on their full duties once more. I won't have to wait for their population to be reestablished.
RADAR