An aquarium heater is a device designed to warm water and maintain it at a set temperature. Temperature is one of the most critical environmental parameters and the most variable factor to consider when creating comfortable conditions for fish and plants.
Since most aquarium fish species originate from tropical regions, they prefer temperatures of 24-28°C, which is higher than typical room temperature in our homes, especially during fall and winter. If you plan to keep tropical fish, an aquarium heater becomes essential equipment.
Types of Aquarium Heaters
Submersible Coil Heaters
The most common type is the submersible aquarium heater with a nichrome coil. The heating element consists of a high-resistance wire coil heated by electric current. Power ratings range from 25 to 300 watts.
In my experience, the most reliable models are those where the space between the coil and glass tube is filled with quartz sand. Sand improves heat transfer and prevents localized overheating of the glass. Air-filled heaters work less efficiently since air is a poor heat conductor.
Open designs require strict water level monitoring—the neck must remain above the surface while the heating element stays completely submerged. If the coil ends up at the water-air boundary, uneven heating occurs and the tube will crack.
Sealed designs can be fully submerged up to one meter deep. This is a safer and more convenient option. Nevertheless, during water changes, you must unplug the heater and wait for it to cool before draining water.
Heating Cables
Heating cables are laid in the substrate and attached to the aquarium bottom with suction cups. Avoid direct contact between the cable and the glass bottom.
This placement creates natural convection—warm water rises, cold water sinks. This benefits plant root systems and prevents water stagnation in the substrate. The drawback is replacement difficulty if it fails, as you’ll need to restart the entire aquarium.
Heating Mats (Pads)
Heating mats are placed under the aquarium bottom externally. Their dimensions should match the bottom dimensions. Such devices typically come bundled with the aquarium or are custom-made.
The advantage is they don’t occupy space inside the aquarium and don’t compromise the aesthetic of your underwater landscape. The disadvantage is replacement difficulty and relatively high cost.
Flow-Through and Integrated Systems
External flow-through heaters exist, as well as models built into external filters. Water passing through the filter gets simultaneously cleaned and heated.
Most external heating systems were created to preserve aquarium aesthetics. However, they’re complex to operate and expensive. Moreover, modern internal heaters are compact enough and easily concealed behind plants or decorations. In my opinion, for most hobbyist aquariums, the juice isn’t worth the squeeze.
Calculating Required Heater Power
For proper power selection, use this simple rule: 1 watt per 1 liter of water with a temperature difference between water and air of about 5-7°C.
A 100-liter aquarium requires a 100-watt heater. However, I recommend installing two half-power heaters instead of one:
- If one non-regulated heater fails, the second prevents temperature from dropping to critical levels
- If the thermostat breaks and the heater “sticks” in the on position, half power won’t cause overheating and fish death
For aquariums with thermostats, you can choose power with margin—1.5 watts per liter.
Aquarium Heater with Thermostat
Modern heaters most often come with built-in thermostats—this significantly simplifies maintaining stable temperature.
Bimetallic Thermostats
The operating principle is based on a bimetallic strip that bends when heated and breaks the contact, then closes the circuit when cooling.
Such thermostats are typically built into the heater tube, with the temperature regulator mounted on the sealing cap. Temperature range: 18-38°C, maintenance accuracy: ±1°C. This is the optimal choice for most aquarists in terms of price-quality ratio.
Electronic Thermostats
Electronic circuits are made as a separate unit located outside the aquarium. They require a semiconductor temperature sensor placed about 5 cm from the heater.
Such systems can maintain temperature with 0.1°C accuracy. For hobby aquaristics, such precision is excessive, and electronic thermostat prices are substantially higher. Additionally, you need to run an extra wire to the sensor inside the aquarium.
Proper Installation and Placement
Place your aquarium heater in an area with good water circulation. Although the heater itself creates slight current through rising warm water, I recommend installing it near the filter outlet or air stone—this ensures more even heat distribution throughout the volume.
Basic safety rules:
- Always unplug the heater before water changes
- Wait for complete cooling before removing from water
- Turn on only after the heating element is fully submerged
- Regularly check tube integrity and connection seals
- Use the heater strictly as intended and according to manufacturer instructions
When choosing an aquarium, immediately consider the need for heating equipment placement. For small aquariums up to 50 liters, a single compact 50-watt heater suffices; for larger systems, plan for two units. If you’re setting up an aquarium for labyrinth fish like bettas or gouramis, remember they’re tropical species requiring stable temperatures around 26-28°C, making a reliable heater absolutely essential.
Historical Note: Obsolete Heating Methods
Salt Water Heaters
Salt heaters were once widespread, working by passing current through saline solution. They were reliable and safe, easily made at home. However, the need to adjust solution concentration led to their replacement by factory-produced coil heaters.
DIY Resistive Heaters
Aquarist radio enthusiasts used high-wattage resistors placed in test tubes. Free space was filled with quartz sand, the neck sealed with compound. This principle, incidentally, is still used in modern industrial models.
Open Flame Heating
Among truly ancient methods, small dry alcohol burners for transport aquariums deserve mention. During winter fish transport, such systems were indispensable, though extremely inconvenient.
Conclusions and Recommendations
An aquarium heater is critically important equipment for keeping tropical fish. When choosing, follow these principles:
- Power: 1-1.5 watts per liter of water
- Quantity: two half-power heaters are more reliable than one
- Type: for most cases, a submersible heater with bimetallic thermostat is optimal
- Quality: choose models with sand-filled tubes
- Placement: in areas with good water circulation
A properly selected and installed heater will ensure stable temperature and comfortable conditions for your pets. Regularly check equipment functionality and follow safety rules when maintaining your aquarium.
Article updated: 2026-09-29
