Not every aquarium is situated in a heated living space like your cozy living room. The temperature can be as low as 12 - 14 °C at night in some rooms of the house or apartment. In the aquarium we usually have temperatures between 23-28 °C. This difference in temperature cools the aquarium water and forces the heater to work continuously.
A rule of thumb in aquatics is that 1 W heat output is ideal for 1 litre of aquarium water. Some “pound stretchers" swear by just 0.5 W heating power for 1 l aquarium water, but this does not save a cent. Their rule of thumb presupposes that a living space is heated and does not drop significantly below 20°C. Otherwise, the heater will not be able to withstand the cold coming into the room from outside and the aquarium will cool down even though it is being heated.
A common reason for not reaching the target temperature despite a warm indoor climate is that the water around the heater is not circulating properly. The heat remains around the heater, which registers its target temperature and switches itself off. The heater has no way of knowing that it is 3-4 °C colder in the rest of the aquarium. That’s why the heater always needs to be installed in a location with good water circulation. This way the heat is evenly distributed in the aquarium.
Of course, the simplest solution is to install a heater with more power. Seen over the year this heater does not consume more electricity than the smaller model (it takes a shorter time to heat the water), but often does not fit into the aquarium lengthwise. And the acquisition costs for this solution hardly pay off. If you have already installed a heater, this can only be an emergency solution.
One thing before we continue: Your fish and plants have no problem with the fact that the temperature in winter is 3 °C lower than in summer. However, the temperature should not drop any further, as the plants and the metabolism of the fish especially are tremendously affected.
A cheap and easy solution is to cover (to insulate) the aquarium on all its outer walls with polystyrene and to put the aquarium on a foam pad ( JBL AquaPad ). An aquarium cover ensures that the heat of the lighting remains in the aquarium and that no heat is wasted through the water surface (it’s the same effect as a hat on your head in winter). Thus, the lighting, in addition to the heater, actively contributes to the heating of the aquarium. A undergravel heating is not suitable for heating the water, as it is often assumed, but merely benefits the climate in the substrate and the circulation of nutrients.
Whether you decide to insulate or heat more, the solutions are simple. Just check in wintertime that the temperature you set can be maintained and if not, be ready to act.