A heater is a vital part in keeping you fish healthy. Fish just like humans don't like to be either too cold or too hot. A heater is designed to keep the water at a constant temperature.
Heaters come in 2 forms:
• submersible - Heaters that can be completely under water
• non-submersible - The tube that encases the heater is the ONLY part
that can be in the water.
I would recommend a submersible heater for the fact that you don't have to watch the water level as closely as with a non-submersible heater.
To determine how long your heater needs to be will depend on how tall you tank is. Just measure from the top of the water line to the top of the rocks and buy a long enough heater with the substantial amount of WATTS.
To determine how many WATTS your heater needs to be you need to calculate 3 to 5 WATTS per gallon. I multiple 5 WATTS x total gallons and get the next highest rating.
(for example) 5 Watts x 26 gallons = 130 WATT heater. Since heaters usually
come in increments of 50 WATTS I would choose a 150 WATT heater. Here's the exception, If the tempurature of the water is 10 to 15 degrees higher than the tempurature where the tank is located I would recommend getting a heater
twice the size. In the case of my example, a 200 WATT heater will be sufficient.
Remember: The smaller the heater the more work it has to do to maintain a constant tempurature which in turn makes it less reliable.