Aquarium Lighting Basics
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Light is the engine that drives plant growth, so getting your lighting right is one of the most important parts of keeping a planted tank. Yet lighting is also where many beginners go wrong, either by assuming brighter is always better or by leaving lights on far too long. Understanding a couple of key ideas, PAR and photoperiod, takes most of the mystery out of it.
PAR stands for photosynthetically active radiation, and it refers to the portion of light that plants can actually use for photosynthesis. Unlike simple brightness as your eye perceives it, PAR measures the useful light reaching your plants at a given depth. This matters because light intensity drops sharply the deeper it travels through water, so a fixture that delivers plenty of usable light to a foreground carpet may deliver much less to plants in a tall tank.
Different plants have different PAR needs. Low-light species such as anubias, java fern, and many crypts thrive under modest lighting, while demanding carpeting and red plants require high PAR to grow densely and color up. Matching your light level to your plant selection is essential; giving low-light plants intense lighting without matching CO2 and nutrients is a fast track to an algae problem.
Photoperiod simply means how long your lights are on each day. For most planted tanks, a photoperiod in the range of six to eight hours is a sensible starting point. Longer is not better; extended lighting mainly gives algae more time to grow and rarely benefits the plants once they have had enough light to photosynthesize for the day. A consistent daily schedule, ideally on a timer, keeps the tank stable.
Light is one leg of a three-legged stool alongside carbon and nutrients, and problems almost always come from imbalance between them. If you increase light intensity or duration, your plants will demand more CO2 and fertilizer to keep up, and any shortfall shows up as algae. This is why turning the light down is often the first and most effective response to an algae outbreak.
Start conservative and adjust slowly. Begin with a moderate photoperiod and a light level suited to your easiest plants, watch how the tank responds over a couple of weeks, and change only one thing at a time. A timer for consistency, a photoperiod that matches your plants, and light intensity balanced against your other inputs will give you healthy growth without feeding algae.