Buying advice
Aquarium Lighting Guide: How to Choose the Right Light
Aquarium lighting gets easier once you stop shopping by gallon count.
A light needs to cover the tank from end to end and deliver enough intensity at the depth where the plants are growing. A shallow 20-gallon tank and a tall 20-gallon tank can have very different lighting needs even though they hold roughly the same amount of water.
For a fish-only aquarium, the job is simpler. You need enough light to see the fish and make the tank look good. For a planted aquarium, plant choice, tank depth, fixture coverage and light intensity start to matter much more.
More light is not automatically better.
Start with the type of aquarium
| Aquarium setup | What matters most |
|---|---|
| Fish-only | Good coverage, pleasing color, timer support and enough brightness to see the tank clearly |
| Low-tech planted | Even coverage and moderate plant-capable output without excessive intensity |
| Moderately planted | Better intensity, useful dimming and enough output at the substrate |
| Higher-light planted | Strong output, good spread, dimming and preferably useful PAR data |
Fish-only tanks usually don't need a powerful planted-tank fixture. If there are no live plants to support, extra intensity mostly gives you more light to manage.
A low-tech planted tank is different. These aquariums usually contain plants that can grow under modest light and don't depend on injected CO2. The goal is enough light for steady growth, not the maximum output the tank can tolerate.
Higher-light planted aquariums put more demand on the fixture. Carpeting plants, dense planting and layouts where compact growth or stronger coloration is the goal may need substantially more light at plant level.
Increasing the light also increases what you're asking from the rest of the aquarium. A tank receiving strong light can become less forgiving when plant nutrition or available CO2 can't keep up.
Match the light to the tank length
Start with the actual tank length.
If the aquarium is about 36 inches long, look for a fixture designed to mount across a 36-inch tank or a size range that includes 36 inches. Don't assume a powerful 24-inch fixture is a good substitute just because its wattage looks high.
Coverage matters.
A short fixture can produce a bright area directly underneath it while leaving both ends noticeably darker. That may be fine for a deliberately shaded layout, but it isn't the same as evenly lighting the full tank.
Many aquarium lights use extendable mounting legs and are sold for a range of tank lengths. Check the actual mounting range of the exact model before buying. Product names such as "36-inch light" aren't a substitute for the fixture specifications.
Pendant and suspended lights work a little differently because mounting height affects their spread. Raising a fixture can improve coverage, but it also reduces the intensity reaching the aquarium.
Tank depth changes the lighting requirement
The distance between the light and the plants matters.
Plants attached near the surface may receive plenty of light while carpeting plants at the bottom receive much less. This is why the height of the aquarium can matter more than its gallon rating when choosing a planted-tank light.
All else being equal, a tall aquarium needs more lighting output than a shallow aquarium to produce the same intensity at the substrate.
Mounting height adds to that distance. A light suspended several inches above an open-top aquarium has farther to travel than the same fixture resting close to the rim.
This doesn't mean every deep tank needs an expensive high-output fixture. It means the useful question isn't simply "How many gallons is the tank?"
It's "How much light reaches the plants?"
How much light intensity do you actually need?
For a fish-only aquarium, there usually isn't a useful PAR target. Pick a light that gives the aquarium the appearance you want without making the tank unnecessarily bright.
For planted tanks, intensity becomes part of the growing environment.
Low-tech tanks generally benefit from restraint. If the plants are growing well, there is little reason to replace a moderate light with a much stronger one just to run the new fixture at full power.
Higher-light setups need more output, especially when demanding plants are growing near the substrate. This is where published PAR measurements can become useful.
The catch is that one intensity number doesn't describe an entire aquarium.
The center of a tank can receive substantially more light than the corners. The same fixture will also produce different readings at different distances. A PAR chart or map showing several positions and a stated measurement distance is much more useful than a product page that advertises a single PAR number with no context.
PAR, lumens and watts
Lighting specifications become confusing because manufacturers may give shoppers several numbers that sound as if they measure the same thing.
They don't.
| Measurement | What it tells you | How useful is it when shopping? |
|---|---|---|
| PAR / PPFD | The intensity of photosynthetically useful light reaching a particular point | Very useful for planted tanks when the measurement distance is known |
| Lumens | Visible light output weighted around human vision | Useful for judging apparent brightness, less useful for comparing plant-growing ability |
| Watts | How much electrical power the light uses | Useful for power consumption, poor as a direct measure of plant-light intensity |
| Fixture length | The physical size or mounting range of the light | One of the first specifications to check |
| PAR map | How plant-light intensity changes across an area | More useful than a single center PAR reading when available |
PAR is the specification planted-tank shoppers usually care about most, but even PAR needs context.
What aquarium hobbyists commonly call a PAR reading is normally PPFD, measured in micromoles per square meter per second. You don't need to memorize the unit to shop for a light. What matters is where the measurement was taken.
A reading measured close to a fixture can't be compared directly with another reading taken much farther away.
Look for the measurement distance. Better yet, look for several measurements across the illuminated area.
Lumens answer a different question. They are useful when you want to know how bright a light appears, but plants don't perceive light the same way people do. Two fixtures with similar lumen output can distribute their light differently and may not produce the same PAR at the bottom of an aquarium.
Watts are even easier to misuse.
A 30-watt fixture isn't automatically twice as bright as a 15-watt fixture, and it doesn't automatically produce twice the PAR. LED efficiency, spectrum, optics, fixture design and how the LEDs are driven all affect the final output.
Use wattage for electrical consumption. Don't use it as your main method of sizing an LED aquarium light.
Low-tech planted tanks don't need maximum output
A lot of planted aquariums don't need an expensive high-output light.
If the tank is relatively shallow and you're growing plants that tolerate moderate or lower light, a basic planted LED may do the job perfectly well.
This is one place where buying more light than you need can work against you. A powerful fixture doesn't become useful simply because it has a higher maximum setting.
Dimming can make a stronger fixture workable, but paying for unused output isn't always a good use of the budget.
Spend the extra money when it buys something the tank actually needs, such as better coverage across a wide aquarium, enough intensity for a deep tank, useful dimming, better mounting options or stronger output for demanding plants.
Otherwise, simple is fine.
Timers and dimming are worth having
A timer removes one daily variable from the aquarium.
The light turns on and off on a consistent schedule instead of depending on someone remembering the switch every morning and evening. A fixture doesn't need a phone app to accomplish this. An ordinary external timer can be enough if the light works properly when power is restored.
Check that behavior before buying a timer for a light with electronic controls. Some fixtures remember their previous setting after losing power. Others don't.
Built-in scheduling is convenient, especially if the fixture also supports gradual changes in intensity.
Dimming is even more useful on planted tanks.
It gives you room to buy a fixture with enough output without being forced to run it at maximum intensity. If the tank receives more light than the plants need, you can turn the fixture down rather than replacing it.
Dimming also makes future changes easier. A lightly planted aquarium can start at a lower intensity, then the lighting can be adjusted as plant mass and the rest of the setup change.
The maximum setting isn't a goal.
When a cheap aquarium light is enough
An inexpensive LED can be a perfectly sensible choice for a fish-only tank.
The same can be true for a shallow, low-tech planted aquarium if the fixture provides reasonable coverage and enough output for the plants being grown.
You may not need app control, individual color channels, sunrise effects or detailed programming. None of those features fixes a fixture that is the wrong size or too weak at the substrate.
Pay first for the light the aquarium needs.
Features come after that.
The case for spending more becomes stronger with deep tanks, demanding planted layouts, unusual tank dimensions or situations where precise intensity control matters. Better fixtures may also provide more useful output data, wider adjustment ranges and mounting systems that make it easier to get even coverage.
A better way to shop for an aquarium light
Before comparing brands, write down the tank's actual length and height.
Then decide what will be growing in it.
For a fish-only aquarium, concentrate on fit, coverage and how the tank will look. For a planted tank, check whether the fixture is intended to support live plants and look for useful intensity information at a distance reasonably close to your tank depth.
If PAR measurements are available, check the measurement distance instead of looking only at the largest number on the page.
Then look at dimming and timer controls.
Price comes after compatibility.
A heavily discounted light that doesn't cover the tank or can't provide enough output at the substrate isn't much of a deal.
Lighting deals make more sense once you know what size and type of fixture you're looking for. Compare offers that actually fit the aquarium rather than choosing the largest discount first.
A basic light can be the right buy for a fish-only or low-tech setup. A planted aquarium with more demanding lighting needs should be compared on usable output, coverage and controls before price.
The useful filter is simple: tank dimensions first, plants second, fixture specifications after that. The brand name and sale price can't make the wrong light fit the tank.