process
Algae in the aquarium
Algae are not a disease and cannot be eliminated. A stable, well-planted tank keeps them at a level nobody notices. Persistent outbreaks almost always trace to too much light for the available carbon and nutrients, to an immature tank, or to accumulating organics.
In one sentence
Algae grow where light, nutrients and carbon are out of balance. Which type appears is a useful diagnostic, because different algae are associated with different imbalances.
Reading the type
| Type | Appearance | Usually associated with |
|---|---|---|
| Diatoms | Brown dust on glass and leaves | New tanks; usually resolves as the tank matures |
| Green spot | Hard green dots on glass and old leaves | Low phosphate, high light |
| Green dust | Fine green film on glass | High light, young tank |
| Hair / thread | Long green strands | Excess light for available carbon; often after a CO₂ interruption |
| Black brush | Dark tufts on leaf edges and hardscape | Unstable CO₂, high organics, low flow |
| Blue-green film | Slimy dark sheet with a distinct smell | Not an alga — a cyanobacterium. Low flow, high organics |
| Green water | Water itself turns green | Free-floating algae bloom; often after a big substrate disturbance |
What actually works
- Reduce the photoperiod before increasing anything. Six to eight hours is plenty.
- Improve flow and remove accumulated detritus.
- Increase water changes for a few weeks.
- Add fast-growing plants to compete for nutrients.
- Manual removal of what you can reach, consistently, every week.
- For green water specifically, a UV clarifier or three days of complete darkness both work where water changes do not.
What rarely works
Buying an 'algae eater' as a first response. It treats the symptom, adds bioload, and in several common cases — the Chinese algae eater and the common pleco especially — creates a larger problem than the algae. Fix the imbalance first and use grazers as part of a stable system, not as a rescue.