Photoperiod

Photoperiod is the length of time aquarium lights are switched on each day, usually set with a timer or a light's built in schedule so it stays consistent from one day to the next. In a planted tank, photoperiod works together with light intensity and CO2 to control how fast plants grow and how much energy is available for algae to use as well.

Most planted tanks run a photoperiod of around six to eight hours, which is generally enough for healthy plant growth without giving algae the extended light window it needs to get established. Some aquarists use a split photoperiod, with a short midday break in the lighting, as a way to disrupt algae growth cycles without shortening the total hours plants receive. Newly cycled tanks are often started on the shorter end of that range and increased gradually as plants fill in and compete more effectively with algae for available nutrients and light.

A consistent schedule matters more than the exact number of hours chosen, since irregular lighting can stress both plants and fish. A common mistake is extending the photoperiod to try to speed up plant growth, which usually feeds algae faster than it helps the plants and is one of the more common causes of an algae outbreak in an otherwise healthy tank.