Algae is a symptom, not an enemy
Algae in an aquarium is not an infestation that arrives from outside. It is the tank telling you that the balance of light and nutrients has tipped in its favour. Scrub all you like; until you change that balance, it comes back. So change the balance.
Reading time ~8 minutes · Builds on Water Chemistry
What algae actually needs
Algae are photosynthetic organisms, and like all plants they grow when they have light and dissolved nutrients — principally nitrogen (as nitrate and ammonia) and phosphorus (as phosphate). The science of how nutrient and light availability govern algal growth is the foundation of aquatic ecology: in any body of water, growth is limited by whichever resource runs out first.[10] An aquarium is simply a very small body of water in which the keeper controls both inputs.
The three levers
Everything you can do about algae reduces to adjusting one of three things:
- Light — too long a photoperiod or too intense a light favours algae. Most planted tanks need only 6–8 hours; a tank with no live plants needs less still. Direct sunlight on the glass is a classic, stubborn cause.
- Nutrients — excess nitrate and phosphate are fuel. Their commonest source is overfeeding and overstocking, with the surplus ending up as dissolved waste. Regular water changes export those nutrients;[7] restrained feeding stops them building up in the first place.
- Competition — healthy live plants consume the same light and nutrients algae want, and a well-planted tank is one of the most reliable long-term defences, because the plants simply outcompete the algae for resources.[10]
Reading the type of algae
Different algae tell you slightly different things. None is cause for panic; each is a clue.
| Appearance | Common name | Usually points to |
|---|---|---|
| Brown, dusty film on glass and substrate | Diatoms | Common in new tanks; often fades as the system matures. |
| Green dots on glass | Green spot algae | Strong light; often associated with low phosphate. |
| Green, cloudy water | "Green water" (suspended algae) | Excess light and nutrients, sometimes sunlight; an algal bloom in the water column. |
| Long green strands | Hair / thread algae | Surplus light and nutrients together. |
| Dark, tufted growth on edges and equipment | Black beard algae (BBA) | Often linked to unstable CO₂/flow in planted tanks and excess organics. |
| Slimy blue-green sheets, distinctive smell | Cyanobacteria | Strictly a bacterium, not an alga; tied to excess nutrients and poor flow. |
A little algae is normal and even healthy — it is part of a living system, and grazing fish and invertebrates enjoy it. The goal of "algae management" is balance, not sterility. A tank with no algae at all is usually a tank with no life in it.
The method for an algae problem
-
Reduce the photoperiod
Put the lights on a timer and shorten the day. Eliminate direct sunlight on the tank. This alone resolves a surprising number of cases.
-
Cut the nutrient supply at source
Feed less, and less often; remove uneaten food. Overfeeding is the most common upstream cause of an algae bloom, because the surplus becomes the nitrate and phosphate that feed it.
-
Export nutrients with water changes
Increase the frequency or volume of partial water changes for a while to physically remove dissolved nutrients from the system.[7]
-
Remove what is there, and add competition
Manually remove visible algae (scrape, siphon, twirl out thread algae). Consider adding or strengthening live plants so something desirable competes for the same resources.[10]
-
Be patient and consistent
A balance shift takes a couple of weeks to show. Resist the urge to throw chemicals at it; chemical algaecides treat the symptom and can harm plants and filter bacteria while the underlying imbalance remains.
Read next
- Water Chemistry — where the nitrate that feeds algae comes from.
- The Complete Method — the water change that exports nutrients.
- Fish Health — why stable, well-managed water keeps fish well too.