20 · Algae Deep-Dive
Algae is not the enemy — unbalanced algae is. Knowing which type you have tells you exactly what to do.
Type 1 — Green water (suspended algae)
Microscopic algae turn the whole pond pea-soup green. Caused by excess nutrients + light, common in new ponds still cycling.
- Fix: add plants, cut feeding, increase flow. A UV clarifier kills it fast but only masks the cause — use as a short-term tool.
- Beneficial bacteria and barley straw help prevent recurrence.
Type 2 — String / blanket weed
Long filamentous algae that mats on surfaces. Thrives on high nitrate/phosphate and still water.
- Fix: more oxygenators and marginals to out-compete it, hand-pull, barley straw, and ensure good flow. Avoid algaecides that rot it in place (they dump ammonia).
Type 3 — Brown diatom slime
A silty brown coating, often in new ponds or shade. Usually harmless and self-resolves as the ecosystem matures.
Type 4 — Blue-green algae (cyanobacteria) — DANGER
Despite the name, this is bacteria, not true algae. It forms a shimmering, oily, often blue-green or mustard scum that clumps and may smell of rotting grass. It can produce toxins harmful to dogs, livestock, and children.
Take seriously
If you suspect blue-green algae: keep pets and children away, do not let animals drink, and improve circulation/reduce nutrients. Persistent or widespread blooms near drinking water warrant professional/health-department advice. It will not be fixed by barley straw alone.
Diagnosis table
| Appearance | Likely type | First action |
|---|---|---|
| Whole pond green, cloudy | Suspended green algae | Plant more, boost flow, patience |
| Long green threads on surfaces | Blanket weed | Hand-pull + out-compete with plants |
| Brown silty film | Diatoms | Usually self-resolves |
| Oily blue-green scum, bad smell | Cyanobacteria | Restrict access, fix nutrients/flow |
Root cause
Every algae type traces back to too much nutrient + too little competition/flow. Fix those and algae becomes a minor, manageable background presence — not a plague.
Algae diagnosis flowchart
- Green water = free-floating phytoplankton. Cause: excess light + nutrients. Fix: UVC clarifier, shade 50% of surface, add floating plants.
- Blanketweed = green filaments on surfaces. Cause: high nitrate, low flow. Fix: remove manually, increase flow, add barley straw.
- Green carpet on rocks = normal in wildlife ponds. Leave it.
- Brown algae on liner = diatoms, usually harmless. Scrub if unsightly.
Natural algae control (no chemicals)
- Cover 50–60% of surface with plants (water lilies, duckweed, water hawthorn).
- Run UVC clarifier 24/7 in spring and summer.
- Add barley straw early in the season (prevents, not cures).
- Do not overfeed fish.
- Limit direct sun to 4–6 hours.
- Use oxygenating plants aggressively.