Filamentous algae and green water in a garden pond

💡 What are filamentous algae and green water?
🟢 Filamentous algae

Long, thin "strips" or threads of green color, often caught on plants, rocks, decorations, filter materials, hoses or even in surface water currents.

They are visible to the naked eye as "grass tails" in the water.

🟡 Green water

Fine microscopic algae (phytoplankton) scattered throughout the water column.

The result is a "turbid" to bright green appearance of the water, reminiscent of the green color of milk.

Both forms are algal "colonies" - but have different life cycles, different causes and different methods of control.

🧬 Biology and nature of algae
🧪 Filamentous algae

They form filamentous colonies of green algae (Chlorophyta), algal biomass growing on underlying surfaces.

They are most often formed in stagnant water, high nutrient levels, and light imbalances.

🧪 Green water

Microalgae in the water column that float in the water without a solid substrate.

Made up of microscopic cells that multiply massively when there is enough light and nutrients.

🌞 Conditions that support algae

Algae are a response to an excess of energy and nutrients in the ecosystem. Main factors:

🔹 Light

Long daylight hours

Direct sunlight

Lack of shade (few plants)

🔹 Nutrients (mainly nitrogen and phosphorus)

Decomposition of feed and fish waste

Unbalanced filter / poor biological filtration

Insufficient outflow of organic matter

🔹 Water stagnation

Lack of flow

Poor oxygenation

🩺 How algae affect the health of the pond
👍 In moderate amounts

They are part of the food chain

They produce oxygen through photosynthesis

They provide shelter for microorganisms

👎 In excess

They reduce light transmission → stress for plants

They consume oxygen at night → stress for fish

They can release toxins or organic waste

They deteriorate water quality and the appearance of the pond

🧠 Why green water occurs

Green water occurs when micro-organisms decompose massively algae in the water column. It is often caused by:

✔️ high light conditions
✔️ high phosphate and nitrogen concentrations
✔️ excess organic waste (overfeeding!)
✔️ poor filtration / low biological capacity

Green water is an indicator of imbalance, not just an aesthetic problem.

🌱 Why filamentous algae occur

Filamentous algae grow mainly where:

✔️ water stagnates
✔️ there are places with low flow
✔️ they have something to cling to (plants, rocks, decorations)
✔️ there is an excess of organic matter

It is a typical problem of an unbalanced ratio of light vs. nutrients vs. water flow.

🧱 Types of Algae and Their Identification
🟢 Filamentous Algae

Long, "tufted" filaments

Grow at the bottom, on plant roots and on filters

Often confused with decoration

🟡 Green Water (Phytoplankton Bloom)

The water is opaque and uniformly green

No visible filaments — it's a "vertical fog"

Can be distinguished by dissolving it in a glass of water

🛠️ How to fight them

The solution is not just "chemical disposal". A combination of ecological, mechanical and technical measures works best:

⚙️ 1️⃣ Reducing nutrient sources
➤ Proper feeding

✅ Only feed as much as the fish can consume in 30–60 seconds
❌ Never leave leftover food in the water

➤ Regular waste suction

mechanical removal of biological waste from the bottom

removal of decomposing materials

➤ Good filtration

biological filtration (biomedia, biological filters)

mechanical filtration (filter sponges, non-woven fabrics)

⚡ 2️⃣ Increasing water flow

Algae love stagnation.

✔️ use pumps and flow
✔️ correctly position the outlets so that the water circulates evenly
✔️ eliminate "dead zones" without movement

🌞 3️⃣ Proper lighting (shading)

reduce the length of direct sunlight

use plants on the surface (water lily, floating plants)

shade half of the pond during the day

🧪 4️⃣ Chemical and technical solutions
🔹 UV sterilization

destroys microalgae cells in the water column

very effective in green water

does not harm fish if properly dimensioned

Use:
➡️ in green water, UV sterilization is often the fastest solution.

🔹 Ozonizers

oxidize organic molecules

not ideal as a main agent

requires experienced installation and control

🔹 Chemical algae products

chemicals can help with a short-term problem
⚠️ but can damage the filter and microorganisms
➡️ use them as a last resort and only according to the instructions

🧠 How to choose the right UV sterilization
📊 Sizing by water volume
Pond volume Recommended UV power
up to 2m³ 5–10W
2–5m³ 10–20W
5–10m³ 20–40W
>10m³ >40W

📌 Rule: UV must be only one part of the strategy (not the only one).

🍁 Prevention is better than cure
🪸 1️⃣ Balanced filter

combine mechanical, biological and UV elements

🍽️ 2️⃣ Proper feeding

dosage according to season and temperature

🌿 3️⃣ Plants

aquatic plants absorb nutrients

shade the surface → less algae

💧 4️⃣ Water monitoring

regular tests of pH, KH / GH, NO₂, NO₃

quick response to increased values