Walstad Tank Lighting: How Much, and For How Long

The short version

• A dirted Walstad tank wants low to moderate light: roughly 30 to 50 PAR at the substrate, not the 60-plus a high-tech carpet demands.
• Run the light 6 to 8 hours a day on a timer. More hours feeds algae, not plants, because a no-CO2 tank cannot use the surplus.
• A cheap nano clip light covers a 10-gallon of Anubias, Java fern, and crypts. An open-top tank under a hardware-store shop light is the classic Walstad rig.
• Below: the numbers, the photoperiod, what low light actually grows, and the siesta trick for the first weeks.

The fastest way to wreck a Walstad tank is to light it like a high-tech one. A dirted tank runs no injected CO2, so the plants grow only as fast as the carbon in the water and soil lets them. Point 12 hours of bright light at that ceiling and the plants stall, while algae, happy with whatever light it gets, takes the surplus.

So the lighting question for a Walstad tank is not "how bright can I go." It is "how little can I get away with and still grow the plants I want." For most low-tech tanks the answer is 6 to 8 hours a day of low to moderate light, and the rest of this is how to hit that.

How much light a Walstad tank actually wants

Aim for roughly 30 to 50 PAR at the substrate for a mixed low-tech planting. PAR (photosynthetically active radiation) is the number that matters, more than watts or lumens, because it measures the light that actually reaches the plants at the bottom of the water column.

A dimmable full-spectrum LED bar set to a medium output lands in that 30 to 50 PAR band over a standard 10 to 20 gallon, and the dimmer lets you back it off if algae shows. Over a small tank, a nano clip-on LED runs low-light plants in anything up to about 10 gallons, though it is too weak to carpet a light-hungry grass like dwarf hair grass, which wants 40-plus PAR.

Height matters as much as the fixture. The same light is far weaker 16 inches down in a tall tank than 8 inches down in a shallow one, which is one reason a shallow, wide footprint is easier to plant low-tech.

The photoperiod: 6 to 8 hours, and why not more

Put the light on a timer and give the tank 6 to 8 hours a day. This is the single most important lighting setting in a Walstad tank, and it is the one beginners get wrong, usually by leaving the light on 10 or 12 hours because the tank "looks better lit."

Plants in a no-CO2 tank saturate. Past a certain point, more light and more hours buy no more growth, because carbon is the limiting factor, not light. Algae has no such limit. Every hour of light beyond what the plants can use is an hour that feeds algae on the glass and the leaves.

Start a new tank at 6 hours a day and hold there for the first two to three weeks while the plants root in. Once you see new leaves, runners, and stems reaching up, push to 8 hours if you want more growth. If algae appears, the first lever is always hours, then intensity.

What low light will and will not grow

Low light grows a surprising amount, which is the whole appeal of the method. Java fern (Microsorum pteropus), Anubias (Anubias barteri), and crypts like Cryptocoryne wendtii all run happily at low light with no CO2, and all three sit inside the 68 to 82 F band a room-temperature tank holds.

What low light will not do is carpet. A tight foreground like dwarf hair grass or Monte Carlo needs 40-plus PAR at the substrate and usually CO2 to grow flat instead of stretching up toward the light. In a low-tech tank those plants go leggy and thin, then melt. Pick runners and rosettes (dwarf sagittaria, Vallisneria) for the foreground instead.

Floating plants change the math below them. A layer of Amazon frogbit (Limnobium laevigatum) cuts the light reaching the substrate hard, which is useful for shading out algae but means the rooted plants under it get less. Thin the floaters if the plants below start to stall.

Light	Best for	Typical PAR and hours
Nano clip-on LED	low-light plants in up to ~10 gal	low light, 6 to 8 hr timer
Full-spectrum LED bar (dimmable)	10 to 20 gal, low to medium plants	~30 to 50 PAR, 6 to 8 hr
Shop or grow light over open top	fast stem and emergent growth	high output, 6 to 8 hr

The siesta schedule, and the first two weeks

A siesta schedule splits the photoperiod into two blocks with a dark gap in the middle: say 3 hours on, a 3 to 4 hour break, then 3 hours on. The total light stays at 6 hours, but the midday dark lets the CO2 that plants drew down in the morning rebuild before the second block.

It is not required, and plenty of Walstad tanks run fine on one continuous 6 to 8 hour block. The siesta earns its keep mostly as an algae tool: some keepers find the dark break interrupts algae without cutting the plants' total light. If your tank is stable, one block is simpler.

The first two weeks are the fragile window either way. A new dirted tank is leaking ammonia and has no plant mass yet to use the light, so hold the photoperiod at 6 hours and resist the urge to brighten it. The plants will look like they are doing nothing for a week or two. They are rooting. Give them the time.

The honest part: too much light is the number-one Walstad mistake

Almost every algae problem in a Walstad tank traces back to light: too many hours, too much intensity, or both, usually paired with a tank that is not yet grown in. The soil releases nutrients, the plants are not established enough to use them, and bright light for 10 hours hands the surplus straight to algae. The fix is boring and slow: cut the photoperiod to 6 hours, dim the light if you can, add fast floating plants to soak up nutrients, and wait it out.

Lighting is also not something you set once and walk away from, even in a low-tech tank. You still trim hours back when algae creeps in, nudge them up as the plant mass grows and can use more, and account for a tank near a window picking up ambient light through the seasons. A timer automates the schedule. It does not remove the judgment.

Frequently asked questions

How many hours should a Walstad tank light be on?

6 to 8 hours a day on a timer. Start new tanks at 6 hours for the first two to three weeks, then move to 8 if the plants are growing well and algae is under control. Running 10 to 12 hours is the most common cause of algae in a low-tech tank.

Do Walstad tanks need a special grow light?

No. A dirted tank needs low to moderate light, not a high-end fixture. A cheap nano clip light handles a small tank of low-light plants, and a hardware-store LED shop light over an open top is the classic, low-cost Walstad choice. A dimmable full-spectrum bar is worth it mainly for the control the dimmer gives.

Can a Walstad tank get too much light?

Yes, and that is the usual problem, not too little. Because a no-CO2 tank cannot use extra light past a point, surplus light and hours feed algae instead of plants. If you see algae, cut hours before you change anything else.

Will low-light plants grow without CO2 in a Walstad tank?

Yes. Java fern, Anubias, crypts, Vallisneria, and most stems grow at low light with no CO2. What will not grow low-tech is a tight foreground carpet, which needs high PAR and usually CO2 to stay flat instead of stretching toward the surface.

Once the hours are set, the rest of a stable tank is the plant choices and the first-month curve the light feeds into. Match a fixture to the plants you actually want on the build planner, read what each plant needs in the compatibility database, and walk the timeline in the first month of a Walstad tank or the rest of the planted-aquarium guides. If floating plants are your algae-shading tool, floating plants for low-tech tanks and whether the tank needs water changes are the next two to read.