Why does your pond suddenly turn green, emit an unpleasant odor, or show blue-green patches on the surface? Is it dangerous for your fish? For your swimming? Should you treat it immediately or look for the cause of the problem?

If you are asking yourself these questions, you are in the right place. At Foudebassin, we have been supporting private pond owners, lake owners, fishing water bodies, and natural swimming areas for years. And we have noticed one thing: in most cases, cyanobacteria are not the real cause of the problem… they are the consequence.

When your pond sends you a warning signal

It often all starts the same way. In spring, your pond is beautiful. The water is clear, the fish are active, and the plants resume their growth. Then the first warm days arrive.

You first notice a slight discoloration of the water. A few days later, greenish or blue-green patches appear floating on the surface. Fish become more discreet, sometimes coming to breathe just below the surface, especially early in the morning. The water feels heavier, and sometimes an unusual odor appears. At this stage, many simply think they have "algae."

However, in many cases, it is a much more complex phenomenon: a proliferation of cyanobacteria, favored by a biological imbalance that has sometimes been established for several months.

The most important thing to understand is that cyanobacteria do not create the imbalance. They take advantage of an environment that has become favorable to them.

Cyanobacteria: neither plants nor true algae

Despite their nickname "blue-green algae," cyanobacteria are actually bacteria capable of photosynthesis. They have existed for over three billion years and are naturally present in most bodies of water.

In a balanced pond, their presence goes completely unnoticed. The problem arises when their growth becomes explosive.

They can then form:

• blue-green floating mats;

• a green paint-like surface;

• gelatinous clumps;

• sometimes even thick foams pushed by the wind.

Some species also produce toxins that can be problematic for dogs, livestock, and, in some cases, swimmers. That is why a significant proliferation should never be taken lightly.

Why does your pond suddenly become favorable to cyanobacteria?

This is probably the most important question. Contrary to popular belief, cyanobacteria do not appear "by chance." They take advantage of a combination of several factors.

Among the most common:

• very warm water;

• low water renewal;

• insufficient oxygenation;

• a thick layer of sludge;

• accumulated dead leaves;

• fish excrement;

• an excess of nutrients, especially phosphorus;

• stagnant zones.

Taken separately, each of these elements is rarely catastrophic. However, when they accumulate over several weeks, they create an ideal environment for proliferation. In other words, cyanobacteria simply take advantage of a pond that has gradually lost its natural balance.

The real enemy is often invisible: lack of oxygen

When talking about ponds, many immediately think of pH, nitrates, or algae. However, the most important parameter is often dissolved oxygen.

Without oxygen:

• fish have difficulty breathing;

• beneficial bacteria slow down their activity;

• organic matter accumulates;

• the sludge becomes increasingly rich;

• anaerobic fermentations appear.

This is when gases like hydrogen sulfide or methane begin to form, causing bad odors and an environment unfavorable to aquatic life. The problem is that these phenomena remain invisible for a long time. From the surface, the pond sometimes still looks perfectly normal. Yet, at the bottom, the biological balance is already tipping.

🌿 Foudebassin Tip

The larger a pond is, the harder it is for the wind alone to maintain even oxygenation across its entire surface.

That is why large water bodies greatly benefit from a surface aerator capable of continuously keeping the water in motion.

👉 Discover our range of Aquitechnics surface oxygenators, designed for natural ponds, private lakes, and swimming areas.

Sediment: a ticking time bomb

Every leaf that falls into your pond. Every twig. Every wilted flower. Every fish excrement. Every dead algae. Everything ends up at the bottom.

Year after year, this organic matter accumulates and forms a layer of sediment. At first, this sediment is part of the normal functioning of an ecosystem.

But when it becomes too abundant, it consumes a huge amount of oxygen to decompose. If this oxygen is lacking…

…decomposition slows down. The sediment becomes thicker. Phosphorus is gradually released into the water. And cyanobacteria then have a real pantry. Many owners try to treat the cyanobacteria.

Very few try to address the cause: the progressive accumulation of organic matter. Yet this is where the true biological restoration of a water body begins.

Why heatwaves completely change the game

Heatwaves are becoming more frequent. And unfortunately, they create exactly the conditions that cyanobacteria thrive in.

When the water temperature rises:

• Its capacity to retain oxygen naturally decreases;

• Fish breathe more;

• Bacteria consume more oxygen;

• Plants also breathe during the night.

Demand is exploding. Supply is decreasing. The imbalance is accelerating.

It is often the morning after a very hot night that owners discover their fish gathered at the surface or notice a spectacular bloom. And this is usually no coincidence.

(Continued in the second part: how to sustainably restore the balance of a pond, why oxygenation is often the first step, how to use BactoVase, Nautex, and CyanoClear wisely, the most common mistakes, and the best strategies for each type of water body.)