Reef Tank Lighting Schedules and Photoperiod

Light is the engine of a reef tank. It feeds the zooxanthellae inside your corals, and it also feeds every alga in the glass. A good lighting schedule gives corals what they need and gives nuisance algae as little as possible. This guide covers the common pattern, how many hours to run, and how to change a schedule without shocking anything.

The three phases of a reef lighting day

Most reef lights let you build a daily curve. Reef keepers usually split it into three parts. The shape matters more than any single number, because it copies the slow rise and fall of sunlight on a natural reef.

How many hours is right?

A total of 8–10 hours, including the ramps, suits most mixed reefs. A shorter peak of 5–6 hours works well for tanks that fight algae. Longer is rarely better. Corals do not use extra hours the way they use extra nutrients, and algae gladly will.

Keep the schedule the same every day. A fixed timer or controller beats a hand switch. Corals follow a daily rhythm, and a steady rhythm is one more kind of stability.

Introducing new lights or a new schedule

Corals adapt their pigment and symbiont density to the light they receive. A sudden jump can bleach tissue or burn it. Use a slow ramp instead:

• Start new fixtures at roughly 30–50% of your target intensity.
• Raise the peak by about 5–10% each week, or slower for light-sensitive species.
• Place new corals low in the tank first and move them up later.
• Watch for early signs: pale tissue, retracted polyps, or colour darkening that reverses.

The same logic applies after a bulb or panel ages. Output drops slowly over months, and many keepers raise intensity a little to compensate. Do it in small steps.

Spectrum, colour channels and the schedule

Most modern fixtures have separate channels, often blue, white and violet. Blue light penetrates water well and makes fluorescent colours glow. White light adds the broader spectrum that corals use for growth. Many keepers run blue earlier and later in the day and add white only around the peak. This also looks natural to the eye.

Do not chase a look at the cost of the corals. A tank can appear vivid under heavy blue while the corals receive too little usable light. If growth slows or colour fades, check intensity and hours before you blame anything else.

Common lighting mistakes

• Changing hours and intensity in the same week, so no cause can be found.
• Running a light without a timer, so the day length wanders.
• Ignoring a failed channel or a dimming panel for months.
• Leaving a window open to direct sun, which adds uncontrolled light and heat.
• Cranking up a new light on day one because the corals 'look fine' at first.

Corals often look fine for a few days after a light shock. The damage then shows as paling, brown patches or slow tissue loss. That delay is why a written log helps.

The algae link

Algae respond to the same light as corals. If you see hair algae, film algae or cyanobacteria, the photoperiod is one of the first things to review. Shortening the peak by an hour or trimming intensity slightly can help, together with a look at nitrate and phosphate. Light alone rarely causes algae. Light plus spare nutrients does.

Some keepers try a midday break, where lights dim for a few hours. Reports are mixed. If you try it, treat it as a single test and log the result.

Log your lighting like any other parameter

A schedule is only useful if you can tell what changed and when. Write down start time, peak hours, intensity per channel and every adjustment. When colour shifts or algae appears three weeks later, that note is the clue. In ReefDeck you can record each lighting change next to your water tests and see both on one timeline. ReefDeck records and displays your own data. It does not diagnose or prescribe.