Algae: The Uninvited Guest

Algae is not one organism—it is a category of photosynthetic life, from green algae to cyanobacteria (“blue-green algae”). In planted tanks, algae usually signals imbalance: light too high for CO₂ and nutrients, dirty organic load, or new-tank dynamics. Treat causes, not only symptoms.

Quick answer: What is the “golden rule” of algae?

Algae is a symptom. Stabilize CO₂ (if injected), align photoperiod with [PAR], export organics, and verify N/P availability for plants—then mechanical removal and biocontrol work.

Identification and typical drivers

Green spot algae (GSA)

• Look — Hard dots on glass and slow leaves (Anubias).
• Common driver — Low phosphate relative to other inputs.
• Fix — Adjust fertilization; manual scrape; patience—plants must outcompete.

Green dust algae (GDA)

• Look — Film on glass that smears.
• Common driver — New-tank ecology; sometimes imbalance during cycling.
• Fix — Many let it run ~3 weeks, then clean—avoid constant glass wiping that resets succession.

Hair / thread algae

• Look — Filaments on wood and stems.
• Common driver — High light with insufficient/unstable CO₂.
• Fix — Stabilize CO₂, reduce PAR or duration, improve flow.

Black beard algae (BBA)

• Look — Black tufts on edges, driftwood, slow growers.
• Common driver — CO₂ instability or poor distribution.
• Fix — Stabilize injection, clean diffuser, spot-treat cautiously with peroxide/liquid carbon protocols—research dose for your volume and livestock.

Staghorn algae

• Look — Antler-like tufts.
• Common driver — Low CO₂ + high organics (dirty filter, overfeeding).
• Fix — Service filtration, reduce feeding, improve CO₂.

Cyanobacteria (BGA)

• Look — Slimy sheets, earthy smell, peels in layers.
• Common driver — Stagnant flow, organic buildup, sometimes low nitrate relative to other inputs.
• Fix — Increase circulation, manual siphon, blackout protocols; antibiotics are a last resort—understand risks to cycle.

Diatoms (brown algae)

• Look — Dusty brown coating—common in new tanks.
• Common driver — Silicates in tap or new sand.
• Fix — Often self-limits as silicates deplete—OTO snails help clean.

Nutrient balance (big picture)

| Factor | If too high | If too low | | :--- | :--- | :--- | | Light | Hair algae, GDA pressure | Slow plant growth | | CO₂ | Fish stress | BBA, hair algae | | Nitrate | Green water (sometimes) | BGA pressure in some systems | | Phosphate | Rarely alone the issue | GSA on leaves |

Biocontrol helpers

Amano shrimp, nerites, and some fish graze certain algae—none replace parameter discipline.

Common mistakes

• Blackout without fixing CO₂/flow — BGA returns.
• Dosing “algae cures” while ammonia is unstable—fix cycle health first.

Frequently asked questions

Is algae bad for fish?

Moderate growth is normal; thick mats can trap detritus and swing O₂ at night in extreme cases.

Should I starve nutrients to stop algae?

Usually backfires—plants starve first; fix light/CO₂ balance instead.

Does UV fix algae?

Green water (free-floating algae) often responds; attached types need cause-level fixes plus removal.