Short answer: Most freshwater tanks want a plant-capable LED run for 6 to 8 hours a day on a timer, at a color temperature between 6500K and 7500K. Low-light plants thrive under almost any modern fixture; the trouble starts when you run a bright light for ten hours because it doubles as room lighting, and the algae takes the surplus. Light drives plant growth, but it is also the single easiest lever to get wrong.
Three numbers decide whether your light helps the plants or feeds the algae: how bright it is, how long it runs each day, and what color it throws. Get those right for the plants you actually keep and the rest of the hobby gets easier.
What the light is actually doing
Plants build tissue from light, carbon dioxide, and nutrients, and light sets the pace for the other two. Give a plant more light than it has CO2 and fertilizer to match, and it cannot use the surplus, so the energy goes to whatever can: algae. Give it too little, and a plant that needs brightness simply melts away. The fish care less. They read the light as day and night, which keeps their rhythm steady, and a few shy species color up better when the tank is not blazing. Your real audience for a strong light is the plants.
How much light: three tiers
Forget the old watts-per-gallon rule, which was built for fluorescent tubes and means nothing for LEDs. Think in tiers instead, matched to how much work you want to do:
- Low light. A basic LED strip or the light in a starter kit. Grows anubias, java fern, cryptocoryne, and most mosses with no CO2 and little fertilizer. This is where a first planted tank should live.
- Medium light. A dedicated plant LED at moderate output. Opens up carpeting stems, dwarf sword, and faster growth, usually with root tabs and a liquid fertilizer, and often a little added carbon.
- High light. A strong fixture running near full output. Grows red stems and demanding carpets, and it demands pressurized CO2 and a full fertilizer routine to keep up. Skip this until you have run a planted tank for a while.
The jump from low to high is not really about the light; it is about everything the light forces you to add underneath it. A stronger fixture over an unfertilized, CO2-free tank grows algae faster than plants. If you want the simplest route to a green tank, stay in the low tier and lean on easy low-light plants that ask for nothing else.
Photoperiod: hours matter more than brightness
Run the light 6 to 8 hours a day and put it on a plug-in timer so the schedule never depends on you remembering. A shorter, consistent photoperiod favors plants over algae, because plants store energy and ride out the dark while algae wants long, steady exposure to bloom. Ten or twelve hours, usually because the tank light is also the room light, is behind most beginner algae outbreaks.
If you want the tank lit in the evening when you are home, try a split photoperiod: four hours in the morning, a few hours dark, then four hours at night. Plants handle the gap fine, and the midday break starves algae of the long run it prefers. Set the target output and the daily hours for your plants with the planted aquarium lighting calculator, then adjust after a couple of weeks of watching the glass.
Color temperature and spectrum
Color temperature, measured in Kelvin, is the tint of the light. The 6500K to 7500K range reads as clean daylight, grows plants well, and shows fish colors honestly, which is why most planted-tank fixtures sit there. Lower numbers look warm and yellow; higher numbers look cold and blue. Plants photosynthesize across the spectrum, so a daylight LED covers the bases without any special “plant bulb” marketing.
Fixtures heavy on red and blue diodes make reds and greens pop, which looks striking in an aquascape. They do nothing extra for plant health over a good daylight spectrum, so buy them for the look if you like it, not because the box promises faster growth.
Too much light is an algae decision
When algae coats the glass or threads through the plants, light is the first thing to cut, before any bottled treatment. Shorten the photoperiod by an hour or two, drop the intensity if the fixture dims, and move the tank out of any direct sun, which overpowers every aquarium light at an intensity no timer can control. The full playbook for clearing it lives in the guide to getting rid of algae without chemicals, and almost every fix there starts with the light.
The reverse trap catches planted keepers: starve a bright tank of nutrients and the plants stall, leaving the light and nitrate for algae instead. If you run medium or high light, feed the plants to match with a proper fertilizer routine, and work out whether the tank needs added carbon through the question of whether aquarium plants need CO2. Light, CO2, and nutrients rise and fall together.
Choosing and placing a fixture
Buy an LED built for aquariums, sized to sit across the full length of the tank so the light reaches the corners instead of pooling in the middle. A built-in timer or a cheap plug timer is worth more than any premium feature, since the schedule does most of the work. Keep the fixture a few inches off the water on legs or a hanging kit, which spreads the light and keeps condensation off the diodes.
Deeper tanks lose light fast as it travels down through the water, so a tall tank needs a stronger fixture than a shallow one of the same footprint to put the same light on the substrate. Factor the depth in before you assume a budget strip will reach a carpet at the bottom.
Aquarium lighting FAQ
How many hours should an aquarium light be on?
Six to eight hours a day on a timer suits most planted tanks. Push past nine or ten and you feed algae without helping the plants. A fish-only tank needs even less, just enough to view them and hold a day and night rhythm.
Do aquarium plants need a special light?
Low-light plants grow under almost any modern aquarium LED. What matters is a daylight spectrum around 6500K and enough output for the depth of your tank, not a bulb sold specifically as a plant light. Demanding species need a stronger fixture, plus CO2 and fertilizer to use it.
Can I leave the aquarium light on all day?
No. A light left on all day, or worse all night, hands algae a long uninterrupted feed and stresses fish that need darkness to rest. Put the light on a timer and hold the daily total to eight hours or under.
Is sunlight bad for a fish tank?
Direct sun is. It is far brighter than any fixture, swings with the weather, and reliably turns the water green. A little indirect daylight does no harm, but keep the tank out of a window that gets hours of direct afternoon sun.
