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.

While most tropical plants can survive even in waters above 30C / 86F, most tend to have better form and coloration at lower temperatures.

Warmer tanks are also significantly more susceptible to algae, especially Green Dust Algae.

The sweet spot for most tanks is between 22 and 26 C / 72 to 79F.
We keep most of our tanks at 24 C / 75F.

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

They require stable tank conditions to grow well, and where temperature is concerned, cooler tanks (with overall slower plants / microbial / algal metabolism) tend to be more stable. 

Above 26C /79F, it is also slightly less easy to saturate water with CO2 as gas solubility decreases as water temperature rises.

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 tank rim. Depending on your room’s ambient temperature and humidity level, a fan can drop a tank’s temperature by a couple of degrees. Every degree counts, so this option is generally worth a try.
   
   
2. Aquarium Chiller. These tend to be expensive, and can often be the most expensive part of an aquarium setup. They also require a powerful filter to run them, or a separate pump system. They also warm up the surrounding area in the room and the cheaper ones can be noisy.
   
   
3. Air-conditioning. This comes with higher electricity cost, but otherwise offers the most consistency and convenience among the 3 methods. When combined with a fan, you can usually drop tank temperatures a couple of degrees lower than 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 experience significantly slower growth at those temperatures. This range is also outside the general comfort zones of most fully-tropical fish. In some species, it makes them more susceptible to certain diseases like white spot.

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

The sweet spot for most tanks lie 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.