Best Temperature?

Above: In cooler waters ( 22 to 26 C / 72 to 79 F ) Bucephalandra grow markedly better and mosses display richer, greener tones. Above 29 degrees Celsius / 84 F, most aquatic mosses tend to be more yellowish and melt more easily.

Temperature plays a powerful, silent role in planted aquariums.

Above: sensitive plant species such as Hygrophila sp. Chai (above) and Eriocaulons stunt / melt more easily in warmer waters (above 26C / 79F). 

What you can do

In cooler regions, raising temperature is easy as heaters are common and inexpensive.

However in warmer places,  lowering tank temperature is far harder.

There are 3 options, all with certain drawbacks.

1. Fan. There are aquarium-specific designs that sit on the edge of the tank. Depending on the ambient temperature and humidity in your room, a fan can lower the temperature of a tank by a few degrees. Every degree counts, so this option is usually worth a try.
   
   
2. Aquarium Chiller. These tend to be expensive and can often be the most expensive part of an aquarium set-up. They also require a powerful filter to run, or a separate pumping system. They also heat the room around them, and the cheaper ones can be noisy.
   
   
3. Air-conditioning. This will cost more in electricity, but is the most consistent and convenient of the 3 methods. Combined with a fan, you can usually reduce the tank temperature by a few degrees below the actual setting on the air conditioner.

Lighting choice also matters. Metal halide and T5 units hung low contribute much more heat than LED units.

What about cooler temperatures?

While tropical plants can survive below 20C /68F, most species will experience significantly slower growth at these temperatures. This range is also outside the general comfort zone of most fully tropical fish. In some species it makes them more susceptible to certain diseases such as white spot.

Above: In many stem species such as Cabomba furcata, the right temperature helps to develop shorter internodes and a more compact growth form. In warmer waters, they develop more elongated internodes with looser form.

The sweet spot for most tanks lie in between ( 22 to 26 C / 72 to 79 F). This contributes to tank stability, and brings out the best form and coloration in most ornamental aquatic species.