Planted Aquarium Light Calculator: How to Pick Watts
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A light calculator for planted aquarium tanks sounds like a single formula, but the source used for this page does not give one. The Aquarium Co-Op lighting guide says nothing about watts per gallon or PAR values. What it does give is a method: start dim, raise the light slowly, watch for algae, and keep the hours short at first. This page turns that method into steps you can follow on paper, and it checks four lights from the pack on the numbers their own listings state, so you can see which ones fit your tank length.
Quick answer
- There is no wattage formula in the source. It recommends starting at about 20 to 40 percent brightness on a dimmable light and raising it while no algae appears.
- New planted tank: 6 to 8 hours a day. Established tank with growing plants: up to 8 to 12 hours a day. Shorten again if algae gets out of hand.
- Match the light to the tank length first, using the range each listing states.
- Use a timer, and do not leave the light on all night, even in a moonlight mode.
Why a single watt number does not settle it
People want a calculator because watts feel like a simple unit. The trouble is that the listings in this pack show how little watts alone tell you. A 6 watt light and a 48 watt light can both be sold as planted aquarium lights. What matters for your tank is the combination of light output, how far the light sits above the water, how deep the tank is, which plants you grow, and how many hours it runs. The guide used here says to consider the tank’s height when setting intensity, but it gives no numbers for depth, and it says it does not classify medium light with numeric thresholds. Any calculator that promises an exact answer is claiming more than this source supports.
A step-by-step calculation you can do
Step 1: Record your tank length
Each listing below states the tank length range it fits. Write down your tank’s length and rule out lights whose range does not include it. This is the one hard filter the listings give.
Step 2: Note what the light can do
Look for dimming. The guide recommends starting at about 20 to 40 percent brightness. If a light cannot be dimmed, the guide says you can raise it above the tank or cover some LEDs with black electrical tape. A light with stepless dimming from 1 to 100 percent, like the fishkeeper listing, gives you the most room to follow that advice; a light with five fixed brightness levels, like the hygger lights, gives you five steps.
Step 3: Choose the hours
The guide says a new planted tank should get only 6 to 8 hours a day while plants adjust, and an established tank with growing plants can go up to 8 to 12 hours a day. Use a timer so the schedule stays steady. The hygger lights list timer choices of 6, 10 or 12 hours, so the 10 and 12 hour settings suit established tanks, and the 6 hour setting suits a new tank.
Step 4: Change one thing at a time
For low-tech tanks without CO2 injection, the guide says to change one variable at a time and wait two weeks before the next change. If you raise brightness and lengthen the hours in the same week, you will not know which change caused an algae problem.
Step 5: Watch the plants and the glass
The guide says excess light gives algae an advantage, and the goal is to balance light with nutrients and CO2 so that plants outcompete algae. If algae starts, lower the intensity or shorten the hours. A mature tank with large plants may need more light, or the plants may need pruning, per the same guide.
Worked example with the listing numbers
Say your tank is 16 inches long. The hygger 18W is listed for 18 to 24 inch tanks, so it is out. The hygger 14W clip is listed for 12 to 20 inches, so it fits. The WEAVERBIRD is listed for 8 to 15 inches, so it is out. The fishkeeper listing does not state a length range in the data used here, but its two holders raise the lights up to 12.7 inches above the tank for larger tanks, so check the listing for the range before buying. Now apply steps 2 to 4 on the one or two that remain.
The four lights from the pack
These are the lights the pack provides for this keyword. None of the listings includes a coverage chart, so the facts below are the sellers’ stated specifications.
hygger 18W 24/7 Lighting Aquarium LED Light
- Power and output: 18 watts, 1075 lumens, CRI 85.
- Tank length: 18 to 24 inches.
- Modes: a default 24/7 natural mode with orange sunrise, white daylight and a blue moonlight period, and a DIY mode.
- Adjustments: timer of 6, 10 or 12 hours; 5 brightness levels; colors include white, red, blue, green, purple, orange and cyan.
- Other: white 6500K, an extendable bracket, a separable adapter, and a stated lamp bead life of at least 50,000 hours.
Note the default mode: the listing says the light is off outside the programmed periods, with moonlight lasting to about 10:50 pm. That fits the guide’s advice that plants need a dark rest period, although the guide also says not to leave a light on all night even in moonlight mode.
hygger Clip On 24/7 Lighting Aquarium LED Light (14W)
- Power and output: 14 watts, 817 lumens, CRI 85.
- Tank length: 12 to 20 inches.
- Mount: clip bracket for tank walls less than 0.9 inches thick; the body can be installed horizontally or vertically.
- Adjustments: same 6, 10 or 12 hour timer and 5 brightness levels as the 18W.
With only five brightness levels, you cannot dial exactly to the 20 to 40 percent starting point in the guide, but the lowest level is your starting place and you can raise the light higher above the tank if the bracket allows.
fishkeeper Clip-on Planted Aquarium Light, 6 Row LED, 48W
- Power: 48 watts, 4.9 pounds.
- Dimming: 1 to 100 percent stepless.
- Controller: LCD display, customizable timer for daily on and off times, nine built-in colors, a 24/7 auto cycle, and a CR1220 battery that keeps settings during outages.
- Height: two holders lift the lights up to 12.7 inches above the tank.
- Claim: the listing says it delivers over 57 percent higher brightness than many standard clip-on fish tank lights.
The stepless dimming is the best match for the guide’s 20 to 40 percent starting advice. The raised holders also fit the guide’s point about lifting a light to soften it. Because it is the strongest light in the pack by stated wattage, the guide’s caution applies: full brightness can be too strong for some tanks and plants.
WEAVERBIRD 6W 12 LED Planted Clip Lamp
- Power and output: 6 watts, 600 lumens, 8000 Kelvin, 110V US plug.
- Tank length: 8 to 15 inches (20 to 40 cm), glass up to 6 mm thick.
- Controls: none stated for dimming or timers.
The listing does not mention dimming or a timer, so you would need an outlet timer for the schedule in step 3 and would adjust intensity only by distance. That is a limitation to weigh against its small size.
Comparison table
| Light | Watts | Lumens | Tank length stated | Dimming | Timer |
|---|---|---|---|---|---|
| hygger 18W | 18 | 1075 | 18 to 24 in | 5 levels | 6, 10, 12 h |
| hygger clip 14W | 14 | 817 | 12 to 20 in | 5 levels | 6, 10, 12 h |
| fishkeeper 48W | 48 | not stated | not stated | 1 to 100% | Customizable |
| WEAVERBIRD 6W | 6 | 600 | 8 to 15 in | not stated | not stated |
Where to read further
The site has a larger comparison, how to pick the best planted aquarium light, with eleven options, and a page on lights for a small tank, the best aquarium light for a 5 gallon tank.
Mistakes to avoid
- Choosing by watts alone. The listings give watts, lumens and length ranges, and the guide cares more about intensity, hours and algae.
- Running the light at full brightness on day one.
- Leaving the light on all night. The guide says plants need a dark rest period.
- Changing brightness and hours at once.
- Ignoring tank height. The guide says to consider it but gives no numbers.
FAQ
Is there a watts per gallon rule?
The source used here does not give one. It also does not give PAR values or numeric light tiers, so this page does not either.
How many hours should the light run?
The guide says 6 to 8 hours a day for a new planted tank and up to 8 to 12 hours for an established one.
Where should I set the brightness?
The guide suggests starting around 20 to 40 percent on a dimmable light and raising it while no algae appears.
What if my light cannot be dimmed?
The guide says to raise the light above the tank or cover some LEDs with black electrical tape.
Which listed light has the widest tank length range?
Not directly comparable. The hygger 18W lists 18 to 24 inches, the hygger 14W lists 12 to 20 inches and the WEAVERBIRD lists 8 to 15 inches. The fishkeeper listing in this pack does not state a range.
How do I handle algae?
The guide says to lower intensity or shorten the hours, and to change one variable at a time, waiting two weeks between changes in a low-tech tank.
Bottom line
Filter lights by tank length, choose one you can dim and put on a timer, start at 6 to 8 hours and low brightness, and change one setting every two weeks while you watch for algae.
