In your home aquarium, the best location for placing a heater is near the maximum water flow, such as the outlet (or inlet) from the filter, or in the stream of a powerhead. Having water flowing directly past the heater is what quickly and evenly disperses heated water throughout the tank.
Water Heater Size and Location
For most small aquariums in a home that is not very cold, the size of the aquarium heater should be 5 watts per gallon capacity of water. So a 20-gallon aquarium should have a 100-watt heater.
If you have a tank that is over 40 gallons, consider having two heaters placed at opposite ends of the tank; instead of one 200 watt heater, use a 100-watt heater at each end. Your water temperature will be more uniform throughout the tank, and in the event that one heater fails, the tank will not be left completely without heat.
Heat Distribution in a Home Aquarium
There are three ways in which heat can be distributed throughout your tank:
- Conduction: Conduction is the typical spread of heat from hot areas to colder areas, seeking a state of thermal equilibrium. When water is warmed up, the water molecules gain kinetic energy and begin bouncing around more; this energy spreads throughout the water as heat. Water is a decent conductor of heat, but most of your heat transfer in a tank will come from convection and circulation.
- Convection: Convection occurs when the water becomes less dense and rises to the top as it warms, creating convection currents. When your heater warms up the water around it, the water rises to the top and displaces cooler water, which is denser so it sinks lower.
- Circulation: The main vehicle for heat transfer in your aquarium will be from the circulation of water by your filter pump. Ideally, your filter will mix the water thoroughly to prevent any hot or cold zones from persisting in the aquarium.
Place the aquarium heater where there is high water flow, but avoid sucking newly heated water directly into the biofilter where it may overheat your beneficial bacteria. Instead, place the heater near the filter outflow; the warm water from the heater will be pushed along in the circulation current into cooler water.
Place the heater horizontally just above the gravel near the filter discharge (assuming that it is a fully submersible heater). Adding an air stone under the filter intake will also pull denser cool water near the bottom of the aquarium upward with the rising bubbles toward the stream of circulation and into the filter.
It is always advisable to have a fully submersible heater rather than one that is partially submersible. This gives the hobbyist more freedom of placement for whatever heating position the situation calls for. If your heater can only be submerged partially, put an air stone under it to help draw the cool water up to the heat source.
As submersible heaters may not last as long if installed horizontally, it is a good idea to replace most heaters once a year. Only very high priced heaters are made to last for many years; the older the heater, the higher the possibility of failure. Either it may stop working completely or it may never turn off and cook the fish! Heaters are run on thermostats and inexpensive heaters have rudimentary connections. It pays to buy a better quality heater for the safety of your fish.