Thermal stratification of the pond: why fish die from lack of oxygen at the bottom
8.8 mg/L of oxygen at most in 20 °C water
7.3 mg/L at most in 32 °C water
4 to 5 mg/L minimum for warm-water fish (4) and cold-water fish (5)
Before dawn when oxygen is lowest
How a pond stratifies in summer
The sun first warms the surface. Warm water being less dense, it floats on the cooler water. Three layers form: the warm surface layer, stirred by the wind and the richest in oxygen; the thermocline, a transition zone where the temperature drops quickly; the deep layer, cold and dense, which has neither direct contact with the air nor light.
The deeper and more sheltered from the wind the pond is, the more stable this barrier is. In a smaller body of water, stratification can remain temporary: wind and rain can break it up and then let it reform several times during the same summer.
Why the bottom lacks oxygen
Three mechanisms combine at the bottom. The water is no longer in contact with the air, so it receives almost no more oxygen. There is not enough light for plants and algae to produce it. Finally, the decomposition of leaves, excrement, and food remains, that is, the sludge, consumes the remaining oxygen.
Heat worsens the situation: the warmer the water, the less dissolved oxygen it can hold, while fish need more of it.
| Reference | Value | What it means |
|---|---|---|
| Maximum oxygen at 20 °C | about 8.8 mg/L | The ceiling for temperate water |
| Maximum oxygen at 32 °C | about 7.3 mg/L | The ceiling drops as the water warms |
| Minimum, warm-water fish | 4 mg/L | Below this threshold, growth and health are at risk |
| Minimum, cold-water fish | 5 mg/L | Case of trout, which are more demanding |
At night, plants and algae consume oxygen instead of producing it: that is why the lowest measurement rises just before dawn. We detail the right reflexes in the article How to avoid fish mortality in the pond.
The turnover: when the bottom suddenly rises
Strong wind, rain, or a cool night can suddenly mix the layers. The bottom water, which contains decomposing matter, very little oxygen, and toxic decomposition products, disperses throughout the pond. The average oxygen then drops below the threshold that fish can tolerate.
Warning: if the water turns brown, gray, or black, gives off a putrid smell, and the fish come to gulp at the surface, act immediately. Stop feeding, turn on any available aeration (air pump, fountain), and measure the oxygen. Avoid stirring up the sludge.
Warning signs to watch for
- Fish gulping at the surface, especially early in the morning.
- Water turning brown, gray, or black after a storm or strong wind.
- A putrid smell when the bottom water is stirred up.
- A warm and calm period, with thick sludge at the bottom: the ideal conditions for an oxygen-free bottom.
- An oxygen measurement below 4 to 5 mg/L at the end of the night.
To make the risk objective, a drop oxygen test to use at dawn is worth more than assumptions. If you also have doubts about the balance of the water, our free water analysis completes the picture.
Aerating the bottom: how to do it
A diffuser placed at the bottom releases fine bubbles. As they rise, they carry the bottom water to the surface: the water column circulates, comes into contact with the air, and oxygen reaches where the sludge needs it. In small bodies of water, regular mechanical stirring can prevent the formation of stable layers. This circulation also supports the decomposition of the sludge (see how to treat sludge).
| Bottom aeration (diffuser + compressor) | Oxygenator or surface fountain | |
|---|---|---|
| Reaches the bottom | Yes, that is its role | Mostly the surface layer |
| Discretion | Compressor on the bank, nothing visible on the water | Visible device and jets |
| Typical use | Deep and sludge-filled ponds | Shallow bodies of water, decorative effect |
We compare the two approaches in bottom aeration or surface oxygenator.
Caution: in a pond that is already heavily stratified, a sudden start can suddenly bring back the poor bottom water. Common precaution: install the aeration in spring, before stratification, or start a few hours a day before switching to continuous operation.
- Locate the deepest point with a weighted string, that is where oxygen is most lacking.
- Place the diffuser on a flat support, above the sludge rather than in the sludge.
- Run a weighted hose to the bank, with a check valve, and install the compressor in a sheltered, ventilated place, above the water level.
- Run the aeration continuously during the warm season.
- Check the oxygen at dawn at the beginning, to verify that the bottom is breathing.
Which equipment to choose
Bottom aeration consists of a diaphragm compressor, a weighted hose, and one or more diffusers. Check that the compressor's flow rate covers that of the diffusers with a margin: the deeper the water, the more pressure reduces the actual delivered flow. Here is what we use and recommend depending on the case.
20 cm Hi Oxygen Disc Diffuser20 liters per minute, for deep pointsfrom €59.62
OASE SubFlo Diffuser DAir diffuser for deep waterfrom €392.72
HiBlow HP-100100 L/min compressor for pond and lakefrom €488.00
Secoh JDK-S 80Diaphragm compressor, Japanese qualityfrom €319.95
For a pond or a small lake, Secoh kits combine compressor, hose, and diffusers, sized to the water volume:
| Kit | Water volume | Price from |
|---|---|---|
| Kit n°1 | 1 to 10 m³ | €219.00 |
| Kit n°2 | 10 to 20 m³ | €291.48 |
| Kit n°3 | 20 to 40 m³ | €413.10 |
| Kit n°4 | 40 to 60 m³ | €537.81 |
| Kit n°5 | 60 to 80 m³ | €710.81 |
Beyond that, we combine a HiBlow or Secoh compressor and several diffusers. Find the entire range in the aeration kits, diffusers, and compressors collections.
On the deep ponds we equip, we prefer two well-distributed diffusers to one large one, a sheltered compressor with a flow margin, and a start-up in spring. The cheapest aeration is the one that runs before the storm, not after.
Doubt about the sizing of your pond?
Describe its size and depth to us: our online water analysis is free. Free shipping from €149 (excluding bulky products).
Frequently asked questions
What is the thermal stratification of a pond?
It is the separation of the water into layers of different temperatures: a warm layer on the surface, a transition zone (the thermocline), and a cold and dense layer at the bottom. Since the bottom water is no longer in contact with the air, it receives very little oxygen as long as the layers do not mix.
Can a small shallow pond stratify?
Yes, but often temporarily. In a small body of water, a gust of wind or rain can break up the stratification and then let it reform several times during the same summer. A deep pond sheltered from the wind generally keeps more stable layers.
Why do fish die after a storm in summer?
Wind and rain can suddenly mix the layers. The bottom water, poor in oxygen and loaded with decomposition products, then disperses throughout the pond and can make the oxygen drop below the bearable threshold. The water often turns brown, gray, or black, with an unpleasant smell, and the fish come to breathe at the surface.
From what oxygen rate are fish in danger?
For warm-water fish, the University of Iowa indicates a minimum of 4 mg/L for growth, reproduction, and health, and 5 mg/L for cold-water fish like trout. Fry need more. Below these benchmarks, it is better to act without waiting, starting by measuring oxygen at the end of the night.
Does bottom aeration prevent stratification?
In small bodies of water, regular mechanical stirring can prevent the formation of stable layers. A diffuser placed at the bottom brings the water up with the bubbles: the water column circulates, comes into contact with the air, and oxygen reaches the bottom. The result depends on the depth, volume, and installed power.
When should bottom aeration be started?
The best is to install it in spring, before stratification, then let it run continuously during the warm season. In a pond that is already heavily stratified, a common precaution is to start gradually, a few hours a day, before switching to continuous operation.
Sources consulted: IISD Experimental Lakes Area, lake stratification and mixing · Iowa State University Extension, Managing Iowa Fisheries: Water Quality (2008)
To go further
- Natural pond: the guide for healthy water all year round
- The RedOx potential in ponds and basins
- Phosphates in the pond: combine SeDox Max and aeration
The Foudebassin experts, experts since 1998
Les produits conseillés par nos experts
Voir toute la sélection
JDK-S 80 - Top quality air pump - Japan quality - Secoh
€335,95
Voir le produit
JDK-S 60 - Top quality air pump - Japan quality - Secoh
€304,45
Pontec PondoAir Set 1800 - Aeration kit with air pump - Pond up to 20,000 L
€95,67
JDK-S 100 - Top quality air pump - Japan quality - Secoh
€419,95
Voir le produit
