Reducing the Time
If algae is a problem in the tank, a contributing factor is usually too much light. Reducing the time the aquarium lights are on to eight hours, or a bit less if necessary, will help reduce the algae growth.
Some fish prefer lower lighting conditions. Many cichlids thrive on less light, as do many species of the tetra family. If the room has good ambient lighting, it's possible to reduce or even eliminate additional aquarium lighting.
Increasing the Time
If there are live plants in the aquarium, the aquarium lighting period should be increased. Plants require as much as twelve hours per day, however the time will depend on the aquarium setup and species of plants.
Use a Timer
The biggest obstacle in maintaining uniform aquarium lighting is that owners aren't able to turn the lights on and off at the same time each day. Fortunately there is an inexpensive and easy way to remedy that problem.
Purchase an inexpensive timer and plug the lighting unit into it. Set the on and off times to obtain the desired period of light, then cross that task off your daily 'To Do' list permanently. I highly recommend timers for all aquarium owners.