Launder filter and lagooning: can they run out of oxygen? The signs, causes, and solutions

A constructed wetland or lava rock filter is a miniature ecosystem: billions of bacteria attached to the stones transform fish waste, while plants absorb nutrients. But these bacteria breathe. If the water flowing through the filter bed no longer provides enough oxygen, part of the filter switches to anaerobic mode and may then degrade the water instead of purifying it. This is more common than you might think, especially at the end of summer.

Why a filter can run out of oxygen

The bacteria that convert ammonia into nitrites and then nitrates are aerobic: they need dissolved oxygen. Those that break down organic matter also consume a lot of it. In a bed of stones, all this activity is concentrated over a large surface area, and oxygen arrives only through the circulating water. Three situations create a deficit:

• A flow rate that is too low or irregular: the water remains in the filter bed too long and becomes depleted of oxygen before it exits.
• Clogging: sludge blocks the gaps, creates preferential flow paths and areas where water no longer circulates.
• Heat and load: warm water contains less oxygen, and a heavily stocked or overfed pond adds a lot of organic matter to break down.

The telltale signs

The most insidious issue is phosphorus: in the absence of oxygen, sediments can release the phosphate they had bound, and the filter intended to combat algae ends up feeding them.

Solution 1: prefilter the water

A constructed wetland is not designed to receive leaves, coarse waste, and bottom sludge. A skimmer, mechanical prefilter, or settling chamber upstream retains these materials before they clog the filter bed. This is the most effective long-term measure.

Solution 2: a continuous and sufficient flow rate

A biological filter must never stop for long during the season. A pump shutting off for a few hours in midsummer is enough to destabilize the filter bed. Also check the distribution: if the water always emerges in the same place, part of the gravel is not being used. A properly designed drain network distributed across the entire surface is essential.

Solution 3: aerate

Aeration with diffusers enriches the water feeding the filter. Place the diffusers in the pond, in the pumping well, or at the inlet of the constructed wetland, depending on the configuration. In upward-flow constructed wetlands, some installers also place diffusers beneath the filter bed to inject air directly into it.

Solution 4: rinse the filter bed

Once or twice a year, flush the drain network through the cleaning access points, and remove the deposits that accumulate on the surface of the gravel. In advanced cases, it may sometimes be necessary to remove and wash some of the stones. Lava rock cleans easily and then regains all its porosity.

Solution 5: reduce the organic load

Less food, no overcrowding of fish, removal of leaves and sludge: anything that reduces the organic matter to be broken down also reduces the filter's oxygen consumption. Specialized bacteria help break down deposits that accumulate in the pond.

Read also

To support bacterial life in the filter bed: Bacteria in natural swimming pools and constructed wetlands: which ones should you use?.

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Frequently asked questions

— Does a constructed wetland need an air pump?

Not always, but it is valuable protection in summer and in ponds heavily stocked with fish. It offsets nighttime drops in oxygen and hot periods.

— Is an anaerobic area always harmful?

Small oxygen-poor areas can help eliminate some nitrates. But a large anaerobic area that smells of sulfur is a problem that needs to be corrected.

— Can the constructed wetland pump be turned off at night?

Not during the season: oxygen is at its lowest precisely at night. The filter must run continuously.

— Should the lava rock be changed?

Rarely. Rinsing is usually enough. Only heavily clogged stones need to be replaced.

In summary

• A lava rock filter can become anaerobic if the flow decreases, if it becomes clogged, or if the water is warm and heavily loaded.
• A sulfur smell, black stones, and green water are warning signs.
• Pre-filtration, continuous flow, aeration, and regular rinsing solve the problem.
• Less organic matter means less oxygen consumed.

Last updated: September 28, 2026.