Photoperiod: how many hours a planted tank's light should run

What the sources recommend

There is no single research-based number for aquarium photoperiod in the sources cited here, but the guidance from growers and retailers is consistent:

Source	Recommendation
ADA (on Aquasabi)	8 to 10 hours a day, controlled every day by timer; about 6 hours during an algae bloom
Aquasabi, new tanks	start at 6 hours, add half an hour a week; aim for 8–10 h under moderate to strong light, 10–12 h under weaker light
EI Light	if algae appear, check CO2 first, then possibly reduce and adjust the lighting period
Tropica	check for direct sunlight on the tank and for a faulty timer as part of algae control

Sources: ^{[1, 2, 6, 5]}.

Two ideas run through all of them: keep the photoperiod regular, which is why every source assumes a timer, and treat light hours as one of the first things to reduce when algae get ahead of the plants.

Hours and intensity are two halves of one dose

Plants respond to the total light they receive in a day, which horticulture expresses as the daily light integral (DLI). Purdue Extension describes DLI as the PAR received each day as a function of both intensity and duration ^[7]:

DLI (mol m⁻² d⁻¹) = PPFD (µmol m⁻² s⁻¹) × hours × 0.0036

So 100 µmol m⁻² s⁻¹ for 10 hours gives the same daily dose as 125 µmol m⁻² s⁻¹ for 8 hours: 3.6 mol m⁻² d⁻¹. A shorter day at higher intensity and a longer day at lower intensity can deliver the same total. The Aquasabi recommendation of longer hours under weaker light ^[2] follows the same logic. The PAR, PPFD and DLI guide has a table.

New tanks

Aquasabi’s cycling-phase advice is to begin with six hours of light and increase it by half an hour per week once the tank is running stably, until the target of 8–10 hours (or 10–12 under weaker light) is reached ^[2]. The reasoning given is that a new tank’s biology and plant mass are not yet established, so both lighting and fertilizing should start lower and grow with the plants ^[2].

CO2 follows the lights

In a tank with CO2 injection, the gas only makes sense while plants are photosynthesizing. ADA’s guidance is to switch CO2 off when the lights go off, with a valve on a timer ^[3], and its lighting article describes a control timer that switches CO2 with the light and runs aeration while the light is off ^[1].

Blackouts

A blackout, keeping the tank dark for several days, is the extreme version of shortening the photoperiod. Tropica lists a 2–3 day blackout among its algae-control measures, especially for green water ^[5]. Aquasabi gives more detail: shade the tank completely, keep it well aerated, switch CO2 and fertilizer off, and limit the blackout to 7 days in tanks with many stem plants or carpeting plants, or up to two weeks if the plants are mainly shade-tolerant mosses, ferns, Anubias and Cryptocoryne ^[4]. It says the method works against cyanobacteria, most filamentous green algae and algae films but is almost useless against red algae, and that it removes algae without removing the reason they grew ^[4]. See green water and hair and thread algae.