Answer to how much light does an aquarium need is not straightforward. There are average numbers and rules of thumb available, but are they useful? We think that instead of the amount of light, what matters is do plants feel well enough in the fish tank.
A basic rule of thumb is to have:
- 1 to 2 watts of fluorescent bulbs per gallon for plantless aquariums;
- 2 to 5 watts per gallon if your tank has live plants;
- lamps should be left on for 8 to 12 hours per day;
- however, as it sometimes happens, it is a bit more complicated.
First and foremost, fish alone do not care about the amount of light. Not that fish do not need light; they have eyes, and thus, they need light. However, fish could comfortably live by the light coming from the room (unless you live in the darkness).
The light in a fish tank is more critical to aquarium plants so that they can photosynthesize, and to you, so you can view the fish. That means, if you have artificial plants, it does not really matter how intense the bulb is, how many bulbs do you have, and the spectrum of light is not as important either. Some lamps are calibrated for the exact purpose, to enhance the color of fish. They perform less well for photosynthesis of plants, but colors are vivid.
Anyway, you can still use whatever bulbs you have and leave them on for 8 to 10 hours. Leaving them on for more could promote algae growth. If the latter is a problem, you can reduce the amount of time the lamps are on or replace part of water more frequently.
The above rule (that you need 2 to 5 watts per gallon of water) is for fluorescent bulbs. They still dominate the market of aquarium lighting but are rapidly replaced by LEDs. LED lamps are energy efficient and emit the same amount of light from a lower amount of watts. But that is only a part of a problem.
It’s easy to convert how much watts of led you need to receive an equal amount of light than from a fluorescent bulb. A simple way to do it is to divide the amount of watt by four. That means you would need 0.5 to 1.25 watts per gallon of water if you use LED lights. But is it helpful? Only slightly.
Do all fish tanks need the same amount of light?
The required intensity of bulbs in your aquarium depends on several things.
- How long are the lights on? In general, a tank should have lights on for 8 to 12 hours, but then it depends how much watts per gallon do you need. Measuring light in watts per gallon is a weird method, in our opinion. Watts is an instant measure of power or the rate of energy consumption. To calculate the energy your fishtank receives, you need to multiply the power with hours the light is on. For example, if you place a 1-watt lamp and leave it on for 12 hours, your tank will receive the same amount of light it would receive from a 2-watt lamp left on for 6 hours.
- Is your aquarium receiving light from outside? It does, but can you measure how much? Obviously, you do not place an aquarium in the direct sunlight, yet still, does the amount of light change throughout the day? Is it possible to take that into account? Not easily.
- What kind of plants will you have? The amount of light required by plants is not equal from plant to plant, some plants love the shade and some need an immense amount of light. If your fish tank has poor lighting, it does not mean you should set up artificial plants only. You can easily ask for low light plants in a pet store that sells plants.
- How tall is the tank? The light shines upon the surface of the water. The taller the tank is, the less light receives the bottom of the aquarium. A rule of thumb is useful only if you have a “regular” fish tank in a broad sense of the term.
Besides, most fish tanks come with lights already installed in the hood. If you are just starting, we do not recommend you to buy a tank without lights already in. If you’d calculate the number of watts based on the above rule of thumb, you could quickly conclude that your tank needs more light. But does it? For example, out 16-gallon tank has only a single 15W fluorescent bulb, which means it gets approximately one watt per gallon. But that is not a problem because we leave the light on for 12 hours and plants seem to thrive.
We think the rule of thumb is useful only if you have nothing else to start with. No preinstalled lights, no limit to how large fixtures you can place at the top of the tank and other. Then, yes, start with the rule mentioned above of thumb.
After that, set it up for an average amount of time, let’s say 10 hours per day, and see what’s going on. If your plants seem to languish, increase the time and see if any changes occur.
Next in series: How to set up an aquarium.