These are an important element for any aquarium that you will need to control the temperature and consider the actual measurements of the room or space the aquarium will be in. Some houses definitely get lower then many people think in ambient air temperature, for this reason if a fish requires a minimum of 18c or higher a heater is definitely recommended. I partially recommend that as 18c tends to be the minimum for many of the fishes who really wont handle below that.

What size heater?

Generally a good recommendation is the 1 litre per 1 watt e.g. 50 watts for 50 litres. The higher the wattage the less the heater will be on but easier not to go too high.

Remember the thermostat!

Heaters contain an internal thermostat, if you have the heater set to a temperature below that of the water the heater will not be on. It switched on and off like the heating in the house and will not use much electricity if the ambient temperature is above. It is recommended or in many cases vital not to switch off as while water decreases temperature somewhat slowly it can reach too cold for many fishes over night.

Temperature guides

As with all animals they have a temperature range they are adapted to with tolerances within, these are species and sometimes variant specific. I generally use the three traditional categories:

• Coldwater: 18c or below. These are fishes which definitely do not require a heater.
• Temperate: 18-24c.
• Tropical: 24c+.

But all of these categories are ones own definitions. No fish fits to manmade rules. Many might fit between these categories, others a narrower range. For example the gold nugget pleco, Baryancistrus xanthellus requires 28c or higher. Others might cross over these categories with small over laps or large. A big indicator is how widespread or restricted a species is in distribution.

Why a temperature?

Temperature is an important influence on the biology and function of fishes. It is important in a fish being able to perform many physiological functions such as digestion, growth and the immune system. Too high or too low could hinder or entirely prevent many of these systems functioning.

Don’t forget that thermometer!

Thermometers measure the temperature for you, you can get a wide range of different types from digital or analogue. Not all heaters are calibrated and these can help but bare in mind of their accuracy. It’s all difficult to be sure sometimes I know but multiple thermometers help, I prefer to avoid digital, I find them more accurate.

What about seasons?

Many fishes do have some seasonal variation and it is an important part of researching the species you aim on keeping. For some fishes this is vital for their biological functions and worth promoting. It could be like the equivalent of being stuck in a constant summer or a constant winter otherwise.

The oxygen issue

To put it simply as temperature increases oxygen saturation decreases. At higher temperatures more surface movement or circulation is likely required. It also means above around 32c it might be a little more difficult in the home aquarium.

The other thermostat

Most heaters do have that internal thermostat, without they will continue to heat up until they are switched off. Sometimes these are faulty and an external thermostat might be of use. I use Inkbirds but D-D do their own. This is like a fail safe, I set the thermostat just above that of the heaters so to prevent overheating. They also have a low temperature alarm so I know if the heaters break and wont heat the tanks.

Never expose the heater while switched on to air, it can and likely will smoke and over heat. It can break the heater or worse.