An Inexpensive Fish Egg Incubator
The first winter after I moved to this house I noticed that my killifish eggs which normally take 2-3 weeks to hatch were taking much longer. I realized that the basement had gotten colder than my previous fish room, and that was causing the long incubation times. I decided I needed to build an incubator for the fish eggs.
I had an old Styrofoam cooler laying around that would make a nice insulated box. The next problem was what to use for a heater. I had an old aquarium heater that had a burned out heating element. The thermostat seemed OK, so I decided to use that as a starting point.
I figured out I only needed a few watts of power to heat the incubator since it was so well insulated. I opened up the heater and removed the heating element and the ceramic form it was wound around. I attached a 3.3K, 20 watt, wire wound resistor to the old heater element connections.
The 3.3K resistor only draws a load of a bit over 4 watts. I used a much larger wattage resistor than I needed because ON-OFF cycling puts a stress on the resistor. The high power rating gives an extra margin from failure. The modified aquarium heater has been running for over 10 years without a problem.
I stuck the heater in a glass jar filled with dry aquarium gravel. That serves a number of purposes. First, it acts as a thermal mass which reduces the frequency of the heater cycling on and off. Second, it makes a nice base and distributes the heat more evenly, preventing the Styrofoam from melting or even catching fire.
Caution! Only make one of these if you are experienced at working with high voltage electricity to prevent electrical shock and fire hazards.