robphoto has one of the best ideas. It is easy to do and costs little.
An aquarium heater works very well. just get a plastic tub deep enough for you film tanks, fill it with just enough water to NOT float the tanks. Take the time, maybe an hour or so, to adjust the heater to heat to your desired temperature. This will remain the same over time.
Another method which I use for B&W is a chart to show the adjustment in time needed as temp changes up or down. I have used it for years with no problems.