Bottom Heating.
Another approach is to heat the aquarium water from the bottom, beneath the aquarium substrate or under the (glass) bottom. This can be done using a heating cable or a heating mat placed under the aquarium. Which of these two methods turns out to be better in practice is something everyone must decide for themselves. If you are not specially cultivating an underwater garden, or if it is not a Dutch-style aquarium, you do not need such cables at all.
Heating cables are one of the hottest topics for discussion and debate. There are many arguments for and against — plant growth rates (some claim everything grows faster with heaters, others see no difference). If you redo your aquarium every year, there is no point in using cables. However, if your aquarium is designed for a longer period, cables will increase your chances of success.
If plants play an important role in your aquarium, you can take care of substrate heating, thereby creating additional conditions for growth. At the same time, heat itself is not the decisive criterion — it is all about the water rising upwards from the heat. Thanks to this circulation, more oxygen and nutrients reach the plant roots. However, the substrate should not be heated too much — overheating will not only slow down growth and harm the plants, but excessive circulation itself will create unfavorable conditions for the roots. Therefore, if supporting flora is important, you need to combine gentle substrate heating with warmth from a standard heater.
There are several approaches to using heating cables. Dupla recommends using powerful cables (250 W per square meter) spaced 2 cm apart from each other. In this case, you need to use a thermostat. Other companies (Dennerle) recommend using low-power cables that can be left on continuously. Heating cables contain a nichrome wire inside, which acts as the heating element — similar to an electric heating pad spiral. But unlike a heating pad, the heat is not concentrated in one spot, but distributed along the entire length of the cable. Such cables are laid in the substrate to heat it.
Cable heaters placed inside the aquarium itself must be of very high quality. Their casing must be neither old nor brittle. In addition, fish must not be allowed to bite through it. Modern cable heaters are produced in a very elastic and dense silicone rubber casing — a material that handles even sudden temperature changes exceptionally well. The heating mat is placed under the aquarium. Its size must exactly match the bottom dimensions. Cable heaters are attached with suction cups inside the tank, on the bottom, and covered with gravel. Cable heaters and heating mats can only be controlled using an electronic regulator.
Why do we need to heat the substrate? If you use a regular heater to warm the water in the aquarium, even when the water is mixed by a powerhead or aerator, the substrate temperature is several degrees lower than the water temperature. You can verify this by sticking a thermometer into the substrate. And if you do not mix the water, the lower layers near the substrate will be quite cold. Practically, the substrate is at room temperature, especially if there is no thermal insulation under the bottom of the aquarium.
Bottom heating has advantages for plants — the heat generated by the device spreads through the substrate and thus warms the root system of the plants. Following the laws of physics, warmed water rises, and cold water sinks to the bottom to be reheated — water circulation occurs without additional assistance. You can equalize the temperature of the substrate and water using an undergravel filter, but it has many drawbacks when used in a planted aquarium.
Due to improved water circulation, nutrients penetrate the substrate better — ammonia (as a product of organic decomposition), iron, etc. This replenishes the substances absorbed by plant roots from the substrate. Water circulation removes toxic substances from the substrate formed by the decomposition of organics in the soil. These substances can have a harmful effect on plant roots. Providing a chelating environment that holds chemical elements, preventing them from oxidizing.
In this state, they are available to plant roots. Similarly, a reducing environment is created rather than an oxidizing one, which prevents the oxidation of elements. This is especially important for iron, which oxidizes very easily. One of the main advantages of heating cables is that they create horizontal water currents due to uneven substrate heating. This promotes the transport of nutrients to plant roots. Heating cables will provide greater stability in your aquarium.
The problem is that bottom heating always raises the same question: what to do if the heater fails? Drain the water from the aquarium so you can pull the mat out from under it and slide in another? Dig the heating cable out of the bottom substrate and replace it with another?
Application procedure: • Clean the inside of the glass bottom of the aquarium from dust and dirt; • Lay the substrate heating cable along the bottom of the aquarium in uniform loops using the provided suction cups. Caution: the substrate heating cable has 2 meters of cold (non-heating) section on both ends. The transition from the cold section to the heating part is marked with a black stripe on the cable; • Lead both cold sections out of the aquarium and secure them to the transformer terminals. Install the transformer in the aquarium cabinet; • Add bottom substrate to the aquarium; • Then plants are planted, and the aquarium is filled with water. Now the transformer can be plugged into the mains. A green LED indicates flawless operation. If it is not lit, it means either the substrate heating cable is not connected, the connected cable is defective, or the transformer is not plugged into the mains;
Water Temperature Measurement. Aquarium Thermometers.
Using a Heater in an Aquarium. Heater Power, Thermostat. 
