Do betta fish need light?

The short answer

A betta does not need an aquarium light in the way it needs a heater. It needs a day and a night - a regular cycle of light and dark - and a room with windows provides that on its own.

What needs a light is everything else in the tank. Plants have published light requirements; a betta does not. So “do I need a light” is really the question “am I keeping plants,” and the answer to that decides the fixture, the hours and the budget.

What is actually published, and what is not

This is worth stating plainly because it changes how much weight to give anything you read on the subject. We went looking for a published photoperiod for bettas - hours of light per day, from a manufacturer or a major care publisher - and did not find one. Aquarium Co-Op’s betta care guide, which this site cites for temperature and tank size, publishes nothing about lighting at all.

What is published is the plant side, and it is specific. Aquarium Co-Op gives anubias, java fern, java moss and Cryptocoryne wendtii as low light, 10-20 PAR, no CO2; dwarf sagittaria as medium light, 20-35 PAR, no CO2. Those are real numbers you can plan against - our nano plant list carries each of them with its source.

So everything below about hours is our judgment, arrived at from the plant requirements and from what causes algae. We are labeling it that way rather than dressing it up as a published standard, because that is the difference between this page and most of what is written on the subject.

What the light is for

• Plants. The only genuinely non-negotiable reason. A planted tank without adequate light is a tank of slowly dissolving plants.
• You. Not a trivial reason. A tank you can see is a tank you look at, and a keeper who looks at the tank daily catches a stuck heater, a clamped fin or a filter that has stopped.
• A day-night cycle. Fish are not nocturnal or diurnal by accident, and a consistent cycle is better than an erratic one. A room with a window already supplies this.

Notably absent from that list: the fish’s color, health or appetite. Those are temperature, water quality and food questions - our care guide works through all three, and none of them is solved by a brighter light.

How long to run it

Six to eight hours a day, on a timer. That is our figure, and here is the reasoning so you can disagree with it.

The plants we recommend are published at 10-20 PAR, which is low light. Low-light plants do not need a long day to meet their needs. Algae, however, responds to light and nutrients together, and in a nano tank nutrients are always present because the tank is small and you are feeding a fish in it. Every extra hour of light is an hour algae gets and the plants largely do not use.

The commonest lighting mistake we see described is not a light that is too weak. It is a bright fixture running ten or twelve hours over slow-growing plants, which is a fair description of an algae farm.

Darkness matters as much as light

A betta sleeps, and it sleeps at night. Tanks lit late in a living room, or lit by a monitor on the same desk, do not give a clean dark period.

The practical version of this: run the aquarium light on its timer, and do not leave a “moonlight” or blue LED mode on overnight. Blue light is still light. It does nothing for the plants and it removes the dark period, which is the one part of the lighting question where the fish genuinely has a stake.

It also matters when you switch on. A tank going from fully dark to full brightness startles a sleeping fish. If the room gets natural light, timing the fixture to come on after dawn and go off before the room goes dark gives you a soft transition at both ends for free.

The sunlight mistake

A tank on a windowsill is not a tank with good lighting. It is a tank with an uncontrolled light source and an uncontrolled heat source.

The lighting half is an algae problem: direct sun delivers far more energy than any nano fixture, for a duration you do not control, varying by season. The heat half is worse, and it is the same argument this site makes about heater sizing - in 4.3 gallons of water, an afternoon of direct sun moves the temperature quickly, and a tank that swings between 76 and 84°F daily is harder on a fish than one held steadily a degree low.

Move the tank off the sill. It is a free fix and it solves two problems at once.

What to buy, if you are keeping plants

A fixture with a timer function or an outlet timer, and ideally a dimmer, because the one thing you cannot look up is how much PAR reaches your substrate. No nano light manufacturer publishes PAR at any depth - an absence we record on every light we list - so a light you can turn down is a light you can tune by plant response.

Beyond that, the choice is between a simple fixture that keeps low-light plants alive and a programmable one that gives you control you may not need. Our nano light picks compare the two on published specification depth, and what a nano fixture actually delivers covers how to work backwards from plant behavior when the manufacturer publishes nothing.

If you are not keeping plants

Then a light is a convenience rather than equipment, and a cheap one on a timer for a few hours in the evening is entirely sufficient. Do not feel obliged to buy a planted-tank fixture for a tank with no plants in it - it will grow algae and nothing else.

We would still gently push you toward a few plants. Not for the fish’s sake, particularly, but because plants give a betta cover and give you something to look at, and the ones we recommend need almost nothing from you.

What we cannot tell you

Whether your betta prefers a bright tank or a dim one. There is a great deal written about that, very little of it published by anyone with a stated method, and we have run no observations of our own. Our approach is to give you the published plant numbers, label our own judgment as judgment, and leave the rest alone.