Aquarium Lighting Basics

How to set photoperiod and intensity for planted tanks, why a timer helps, and how excess light leads to algae.

Why lighting matters

Light drives plant photosynthesis, so balancing duration and intensity is central to a healthy planted aquarium. Both too little and too much light can lead to algae growth, so the aim is to match light to what the plants can actually use. In a fish-only tank without live plants, lighting is mainly for viewing, and the same risk applies: leaving a strong light on for long periods can still encourage algae on the glass and decor.

Photoperiod

For a newly planted tank, a photoperiod of about 6 to 8 hours per day is recommended while plants adjust to their new surroundings. As the tank establishes, the photoperiod can be increased gradually to roughly 8 to 12 hours per day. Lights should not be left on continuously, because plants need a dark respiration period at night. Increasing the day length step by step, rather than all at once, lets the keeper watch for algae and back off if it appears.

Using a timer

An electrical outlet timer keeps the lights turning on and off at the same time every day, providing consistency regardless of a keeper's schedule. A fixed daily cycle is easier on plants and fish than irregular lighting, and it removes the risk of forgetting to switch the light off, which would otherwise extend the photoperiod and feed algae.

Intensity

A practical starting point is around 20 to 40 percent brightness, increased gradually only if no algae appears. If a fixture lacks dimming, raising it higher above the tank or blocking some of the LEDs reduces the effective intensity. Because intensity and duration both add to the total light the tank receives, the two should be adjusted together rather than maximising one and ignoring the other; a moderate intensity over a moderate photoperiod is easier to balance than strong light for a short time.

Intensity for plant demand

Light intensity is often described in PAR (photosynthetically active radiation). Medium light of roughly 20 to 35 PAR suits most aquarium plants except the most demanding species, while high light of about 40 to 50 PAR or more is needed for highly demanding and carpeting plants.

Algae and excess light

When light duration or intensity exceeds what the plants can use, algae can flourish and take over the tank. Starting low and increasing slowly while watching for algae helps keep the system balanced.