Tap anywhere outside image to close
A bio pond is a swimming pool with no chlorine in it. Instead of chemicals, the water is cleaned by plants, bacteria and gravel — the same way a healthy stream cleans itself. It is a beautiful idea. It is also a demanding one, and it does not suit every site or every family. This page explains both sides honestly.
A normal swimming pool keeps water safe by killing things. You add chlorine, it stays in the water all the time, and anything that enters is destroyed within seconds.
A bio pond works the opposite way round. There is no chlorine, so nothing is being killed. Instead, the water is deliberately kept starved. Algae need food — mainly a nutrient called phosphorus. If you remove the food, algae cannot grow, no matter how much sunlight falls on the water. At the same time, a large population of harmless bacteria living on gravel surfaces breaks down anything organic that enters the water.
So a bio pond is not an untreated pond. It is a carefully engineered system with pumps, filters and a plant room, doing a completely different job to a chlorine pool.
A chlorine pool kills what enters the water. A bio pond removes the food, so nothing can grow in the first place.
Every working bio pond has two zones:
Where you actually swim. Usually 1.4 to 2 metres deep, with a hard, clean surface — tile, stone, or a dark liner. This part can look exactly like a normal pool.
A shallow, gravel-filled, planted bed where the cleaning happens. Typically 0.1 to 0.6 m of water over a gravel layer roughly 300–450 mm deep. Water is pumped through it continuously and returns to the swimming zone clean.
Representative references, not our own executed projects.
The regeneration zone needs to be at least as large as the swimming zone — and twice as large if you want the system to be genuinely stable and low-maintenance. In practice, a bio pond needs one and a half to two times the space of a conventional pool for the same amount of swimming. This is the first thing to check on any site, before anything else is discussed.
Three completely different things get sold under the name "bio pond", and it is worth separating them before going any further.
Built for people to swim in. Plants are functional equipment, not decoration. No fish. This is what this page is about.
Built for fish and for looking at. Nobody swims in it. Completely different design, different filtration, different water quality.
A reed bed used to polish treated wastewater on large developments. Sometimes sold to builders as a "bio pond" for green-rating credit. Not a bathing water body.
This surprises most people, so it is worth being clear about. Fish eat, and fish produce waste. Fish waste is rich in exactly the phosphorus you have spent the entire design trying to remove. Adding fish to a swimming bio pond directly feeds the algae you are trying to prevent — and it also raises a hygiene question nobody wants to think about while swimming.
A well-run bio pond will still attract dragonflies, frogs and birds on its own. That is the biodiversity people actually want. Stocking fish is a different project.
People assume the plants do the cleaning. They help — their roots absorb nutrients, and they keep the gravel bed open and permeable so water can pass through it — but the real work is done by bacteria living as a thin film on the gravel surface. The plants are part of the machine and part of the beauty. They are not the machine on their own.
This matters practically: plants must be cut back and physically removed at the end of each growing season. If dying leaves are left to rot in the water, all the phosphorus the plant absorbed goes straight back in.
| Bio Pond | Conventional Pool | |
|---|---|---|
| How the water is kept safe | Nutrients removed so nothing can grow; bacteria break down waste | A disinfectant kills organisms on contact |
| Protection while you're in it | None in the water itself | Continuous chlorine residual everywhere |
| Space required | 1.5–2× a conventional pool | The pool footprint only |
| Pump running hours | Continuous, 24 hours, low power | Typically 8–12 hours a day |
| Filter media | Gravel and biofilm, plus a phosphorus-binding filter | Sand or glass media |
| Backwashing | None — no water lost to backwash | Regular, with real water loss each time |
| Time to reach full performance | One to two seasons | Days |
| If something goes wrong | Slow to correct — days or weeks | Fast to correct — hours |
| Water appearance | Clear, but darker and softer in tone | Bright blue, from the tile colour |
| Winter | Left full and running all year | Often drained or shut down |
| Chemical storage on site | None | Required, with handling discipline |
| Standards and codes | One national guideline worldwide, and it is advisory | Established codes in every major market |
The single most important row in that table is the second one. A bio pond has no protection in the water while you are swimming in it. Everything else — the space, the slow start-up, the limits on how many people can use it — follows from that one fact.
Four stages, in this order. All four have to be present. Leaving one out is the usual reason a bio pond goes green and stays green.
A leaf floating on the surface is harmless. A leaf that sinks and rots releases phosphorus into the water — and that phosphorus feeds algae for months. So the surface skimmer on a bio pond is far finer than on a normal pool: a curved screen with roughly a 0.3 mm mesh, against a 5 mm basket on a conventional skimmer. The aim is to get organic material out of the system while it is still a leaf.
This is where most bio pond projects in warm climates go wrong. A sand filter is the wrong technology here. In water this biologically active, biofilm and lime cause the sand grains to bind together into lumps, flow drops away far sooner than it would in a chlorinated pool, and the filter needs constant heavy backwashing.
A bead filter is the correct choice. It filters down to around 10 microns, and it does a second job at the same time — the beads themselves become a home for the beneficial bacteria, so the filter is mechanical and biological in one vessel. It resists the clogging that defeats sand, and it pairs well with a variable-speed pump for low running cost.
Water is pushed slowly and evenly through the gravel bed. Bacteria on the gravel convert ammonia to nitrate; plant roots and the gravel itself take up nitrate and phosphate. Even distribution matters enormously — if water finds a shortcut through one part of the bed, the rest becomes a dead zone and the whole system underperforms. Plants going yellow in one corner is usually the first visible symptom of a flow problem.
On a modern, pool-like bio pond, a dedicated phosphorus filter carries mineral granules that lock dissolved phosphate out of the water permanently. This is what allows the regeneration zone to be smaller than a traditional swimming pond would need, and it is the difference between water that is "usually clear" and water that is reliably clear.
Bio ponds at temperate latitudes often run without UV, because the water stays cool enough not to need it. Warmer water does not behave that way: algae grow faster, and organisms a chlorine residual would ordinarily suppress become a live consideration. A correctly sized UV stage on the return line is standard specification on any bio pond built outside a cool-summer climate, not an upgrade. A warm-climate proposal without UV in it is cutting a corner someone will pay for later.
The bacteria doing the cleaning need oxygen. Adequate aeration keeps dissolved oxygen high through the whole depth of the water, not just at the surface, and directly improves how well the biology performs. Fine-bubble and nanobubble systems can add useful margin on larger water bodies — but they are a booster on top of a correctly sized regeneration zone, never a substitute for one.
A conventional pool is judged on chlorine level and pH. A bio pond is judged mainly on how little food is left in the water. These are the guide values from the FLL 2017 guidelines for the swimming area:
| What is measured | Target |
|---|---|
| Ortho-phosphate (algae food) | 0.01 mg/l or less |
| Total phosphorus | 0.01 mg/l or less |
| Ammonia | 0.3 mg/l or less |
| Nitrite | 0.01 mg/l or less |
| Nitrate | 30 mg/l or less |
| pH | 7.0 – 9.0 |
| Conductivity | 1000 µS/cm or less |
Values for the higher-specification pool types (FLL Types IV and V), which is what we would build. Simpler pond-type installations are permitted three times the phosphorus.
Look at the first line. 0.01 mg/l is one part in a hundred million. That is the level of restraint required to keep water clear without a disinfectant — and it explains why every single design decision on a bio pond comes back to keeping phosphorus out.
This is the single variable that decides whether a bio pond becomes a pleasure or a problem, and it is worth being blunt about it.
The technology was developed at latitudes where summer water temperature settles comfortably in the low twenties. The biology performs best below about 26°C. Above that it becomes progressively harder to hold: algae grow faster than the system can strip nutrients, and the shallow regeneration zone — the part actually doing the cleaning — heats faster than the swimming zone and suffers first.
Across the hot belt — much of South and Southeast Asia, the Gulf, sub-Saharan Africa, northern Australia, the American Southwest, southern Europe in high summer — an unshaded water body will sit well above that threshold for months of the year. It is not impossible. It requires serious shading, an oversized regeneration zone, higher circulation rates, UV as standard, and on some sites active cooling. But it is a materially harder engineering problem than the same pool in a cool-summer climate, and it has to be designed and priced as one. Anyone presenting a bio pond in a hot climate as the simple, low-technology, low-cost alternative to a swimming pool is not describing the same thing we are.
The corollary is worth stating too. In exactly those locations, winter performance is excellent — the pool stays alive, planted and attractive through months when a conventional pool would be shut down and sheeted over.
We check all of these before we will quote a bio pond. If several of them fail, our recommendation will be to build something else — and we would rather say so at the drawing stage than after the water goes in.
Everything above is available in some form from anyone selling bio ponds. These next points generally are not, and they are the ones that decide whether an owner ends up happy.
A new bio pond takes one to two seasons to stabilise. Algae blooms during that period are normal and expected, not a defect. Clear, settled water usually arrives in year two or three. If the handover photograph is the only thing that matters to you, this is the wrong product.
Clarity is excellent in a well-run bio pond — you will see the floor easily. But the tone is darker and softer, sometimes faintly green or tea-coloured. That is what natural water looks like. Some people find it more beautiful than blue tile. Others are disappointed, and it is better to find that out now.
In any warm climate this is the first question asked, and it deserves a straight answer rather than a reassuring one. Mosquitoes breed in still water. Continuously circulated water with surface movement, no dead corners and no stagnant margins does not support larvae, and a healthy planted zone hosts the invertebrates that predate on them. But all of that depends on circulation being designed correctly and kept running. It is an engineering requirement, not a promise.
Conventional pools sit inside established codes and familiar consultant practice in every major market. Bio ponds do not. Outside German-speaking Europe there is essentially no national code, and the FLL guidelines everyone works to are advisory rather than statutory. On a private residence that is manageable. On any project that has to satisfy an external consultant, an insurer or a formal handover, it is a real complication and belongs in the first conversation, not the last.
This is the least-known option and often the most sensible one. An existing chlorinated pool can frequently be converted: the shell, skimmer and pump stay, the chemical dosing is removed, and a biological filter, phosphorus filter and UV stage are added in line. Lower cost, lower risk, and it works on a site that could never accommodate a full regeneration zone.
If a proposal in front of you specifies a sand filter for a bio pond, it has not been engineered — it has been assembled from conventional pool parts. Drum filters have the opposite problem: they are built for the heavy waste load of a fish pond and do not work efficiently at the much lighter load a swimming pond produces.
Most clients who ask us about bio ponds are not really asking for chemistry. They are asking for a water body that looks natural instead of looking like a blue tiled tank. Those are two separate decisions, and they are worth taking separately — because you can have the natural appearance without giving up the safety of a disinfectant.
Dark stone or aggregate finish, no contrasting waterline tile, deck-level water, planting running to the edge. It reads as a natural water body. Underneath, it is a conventional pool with UV and a low chlorine residual.
Works on any site in any climate, including terraces and rooftops. No compromise on safety, bather numbers or recovery time.
The same natural appearance, plus a real planted biological loop, bead filtration and a phosphorus filter — running a much reduced disinfectant level rather than none.
You get the ecology, the plants and most of the chemical reduction, while keeping a safety net. For most clients this is the honest sweet spot.
No disinfectant at all. Full regeneration zone, the complete four-stage system, UV, and all the constraints described on this page — on space, on numbers, on patience and on climate.
The right answer for an upland estate or a wilderness property. Rarely the right answer for a hot-climate city garden.
We will tell you which of these three your site actually suits — including when the answer is that it suits none of them, and a conventional pool with a good landscape scheme will make you happier for the next twenty years.
Send us the site, the surroundings and a sample of your fill water. We will tell you honestly whether a bio pond belongs there — and what it would take to do it properly.