Does Aquarium Light Cause Algae? What Every Aquarium Owner Should Know
If you upgraded your aquarium light and a week or two later experienced an outbreak of green algae, know that you are not the only hobbyist to have ever made this experience. It seems intuitive, as algae require light, so why not blame your new light? Truth be known, though, lighting seldom causes algae outbreaks, though it can be a contributing factor; typically, an algae outbreak occurs when light and nutrients become out of sync with plant growth.
When one can understand how the light supply affects plants and nutrient availability, it can prevent them from giving algae the advantage and enjoy an algae-free aquarium.
Why New Lights Seem to Trigger Algae
You're familiar with this one; you put a powerful LED aquarium light on instead of your old fixture. The tank looks great; everything is brighter, plants are vibrant, and things just seem to be "buzzing". A week or two later, green algae has started to grow on your decorations and glass.
From what we've read, it looks like the light's fault and, in fact, it might be; the brighter light may simply highlight that there's an existing problem in the tank. Higher light intensity equals increased photosynthesis. Suddenly, plants may need extra nutrients, more CO2 in planted tanks, etc. If these are not available, then algae will take advantage of this extra "power," and it will grow quicker than the plants.
How Light and Algae Really Interact
Like plants, algae also depend on light for photosynthesis. Algae cannot grow in the absence of light. This isn't all; they also need food such as nitrates, phosphates, and organic matter.
Consider light as the throttle pedal; in the presence of the needed nutrients, the greater the light intensity, the faster the algae grow. However, with nutrient limitations and a healthy planting, the algae will never take hold even in bright conditions. This is the reason that two aquariums under identical lights behave so differently-the health of the planting, balance of nutrients, and maintenance practices make all the difference. Many experienced aquarists are concerned with the bigger picture rather than just dimming the lights as soon as any algae is observed.
How Much Light Is Too Much
In most freshwater aquariums, how long your light stays on is more important than how bright it is. A lot of new aquarists turn their light on for 12-14 hours, thinking that brighter plants are healthier. Aquarium plants indeed need enough light, but extra hours mainly promote the growth of algae.
A good recommendation for daily lighting is:
| Aquarium Type | Lighting Duration |
| Fish only | 6–8 hours |
| Low-tech plant tank | 7–9 hours |
| Medium-tech plant tank | 8–10 hours |
| High-tech plant tank | 8–10 hours |
Use a timer so that the amount of light doesn't accidentally increase. Even just 1 extra hour a day over time can make a noticeable difference in the growth of algae.
Other Factors That Encourage Algae
Lights are normally given all the blame, and while they can be an issue, many other things also play a big part:
- Overfeeding the fish – the unconsumed food breaks down into usable nutrients for the algae.
- Not enough water changes – an increase in the amount of nitrates and phosphates in the water gives the algae an added boost.
- Plants in poor health and/or slow growing – this limits their competition with algae for the available nutrients.
- Dirty conditions – the accumulation of organic matter, filthy filter media, rotting leaves, etc., will always give the algae food.
Here is a reminder of what increases algae:
| Common Mistakes | Result for Algae |
| Leaving lights on too long | Extra energy for algae growth |
| Overfeeding fish | Abundant food source |
| Poor plant growth | No competition for nutrients |
| Not enough water changes | Accumulated nitrates & phosphates |
How to Control Algae Without Dimming Your Lights
A lot of people who are new to this hobby tend to turn the lights completely off or set them on a very low light duration to combat algae. While this works in slowing algae growth for a bit, it doesn't solve the problem, and the plants are also being damaged.
One good way to combat algae is by finding the right balance between lights, nutrients, and plants:
- The light cycle is set for the tank type and plant needs.
- Plants are kept healthy via nutrients and CO2 (if used).
- Water quality is monitored via water changes and cleaning the substrate.
- Feed fish responsibly, removing uneaten food promptly.
When plants are growing well, they compete with algae automatically, which means unwanted growth will be kept under control, and the plants won't suffer.
The Bottom Line
So is my aquarium light to blame for the algae?
Yes and no. Algae do need light to survive, but very rarely does light alone cause outbreaks. It is normally light out of balance with the plants and nutrients. Light shouldn't be thought of as something to fight, but as one element of the aquarium.
Once the lighting, plant growth, nutrient levels, and aquarium maintenance levels are consistent, then it is very easy to have an incredibly bright and algae-free aquarium.
If you would like to see a comprehensive guide on selection, lighting schedules, and controlling algae, then check out my Aquarium Lighting Guide.
