Pond full of algae and cloudy water: how to save your fish
Your fish are staying at the surface, mouths open. The water is green or brown and algae are taking over everything. In this situation, the emergency is not the algae: it is oxygen. Here is what to do right away, followed by how to get the pond back in order without putting your fish at risk.
The 5 emergency steps
- Oxygenate immediately: air pump, water jet, waterfall.
- Stop feeding for 2 to 3 days.
- Test the water: nitrites, ammonia, pH.
- Change 10 to 20% of the water using a water conditioner.
- Avoid any harsh algae treatment.
Why the fish are suffering
During the day, algae produce oxygen. At night, they consume a great deal of it. When there are a lot of them, oxygen levels crash in the early morning. Dying algae decompose and use up even more oxygen, while releasing ammonia. In hot weather, the water already holds less oxygen: the combination becomes dangerous.
Step 1: oxygenate, right now
Switch on an air pump, aim a water jet at the surface and keep the waterfall running continuously. Do not switch anything off at night. Even simply spraying the surface with a garden hose on a shower setting helps in the meantime.
Kit no. 1 for ponds from 1 to 10 m³ - Japan quality - Secoh
Complete air pump kit to quickly oxygenate a pond of 1 to 10 m³. Live price and instant add to cart:
Step 2: stop feeding
Uneaten food rots, and digestion uses up oxygen. Two to three days without food is no problem at all for adult fish.
Step 3: test and change part of the water
Measure nitrites and ammonia. If they are high, replace 10 to 20% of the water, no more, with water treated with a conditioner that neutralises chlorine. Repeat every two or three days if necessary.
6-in-1 Pond Water Quick Test – pH, GH, KH, Chlorine, Nitrites, Nitrates
Quick test to check nitrites, nitrates, pH and KH in one minute. Live price and instant add to cart:
JBL BiotoPond 500ml - Water conditioner to prepare pond water
Water conditioner that neutralises the chlorine in tap water during water changes. Live price and instant add to cart:
Step 4: reduce the algae gradually
The temptation is to treat everything at once. That is the worst mistake: tonnes of algae dying at the same time cause a sudden drop in oxygen. Work in stages:
- Remove part of the blanket weed by hand every day.
- Check the UV clarifier (bulb less than a year old, clean quartz sleeve) to tackle green water.
- Vacuum out the sludge if it is thick.
- An active oxygen product can help clear the water, provided you follow the dosage strictly.
BiOx 1000 ml / 32 000 litres – Active oxygen for ponds – Colombo
Active oxygen for ponds that helps break down organic matter, to be dosed precisely according to volume. Live price and instant add to cart:
Also worth reading
To then tackle the cause of green water, read My pond water is green, what should I do?.
READ THE ARTICLEStep 5: stabilise so it does not come back
- Plant enough to cover a third to half of the surface.
- Check that your filtration is sized for your fish population.
- Feed less, but better.
- Keep the aeration running throughout the warm season.
For fast action on water quality, discover our active oxygen solutions.
Assess how serious it is in a few minutes
Before tackling everything at once, watch what your fish are doing. Their behaviour, the time of day and your test results tell you where to start. If in doubt, treat the situation as an emergency: adding oxygen never harms a pond in trouble.
The first few days: what to monitor
Once the emergency steps are done, the situation can still deteriorate, especially in hot weather. Keep a small logbook: date, water temperature, test results and fish behaviour.
- Early morning: this is when oxygen is at its lowest. Observe your fish before the sun gets high.
- Temperature: the warmer the water, the less oxygen it holds. A rising temperature means you need to step up aeration.
- Nitrites and ammonia: test every day until they are back to zero. Our guide to nitrites in the pond explains what to do step by step.
- The smell of the water: a rotten-egg smell signals decomposing sludge, to be dealt with as soon as the situation has stabilised.
To check your oxygen rather than guess, read our article on how to measure dissolved oxygen.
Aquaoxy 500 OaseQuiet aeration kit with tubing and air stone, for ponds up to 5 m³.Floating pond thermometerSits in the pond so you can read the water temperature at a glance.Pontec PondoMatic V4Vacuum for removing sludge from small ponds.
Set up back-up aeration before summer, not during the crisis. A small air pump stored somewhere dry and tested in spring saves you the hours that matter, especially if the emergency strikes on a heatwave evening or on a day when the main pump breaks down.
Mistakes that make things worse
- Stirring up thick sludge in the middle of a crisis: it releases matter that consumes oxygen.
- Starting to feed again too soon because the fish seem better.
- Cleaning the filter thoroughly: you remove the very bacteria the pond needs.
- Adding several products at once: you will not know what helped, and some work against each other.
- Switching off the aeration at night because of the noise.
Frequently asked questions
Should I drain the pond completely?
No. A full drain-down wipes out the beneficial bacteria and puts the fish under severe stress. Opt for repeated partial water changes instead.
Should I take the fish out?
Only if the water remains dangerous despite oxygenation. In that case, an aerated tub filled with pond water and kept in the shade will protect them temporarily.
Why are my fish dying at night?
This is the classic sign of a lack of oxygen at night. Continuous aeration is the priority.
In summary
- Oxygen first, day and night.
- No food for a few days.
- Partial water changes with a conditioner.
- Reduce the algae gradually, never abruptly.
Sources and references
- Penn State Extension, fact sheets on dissolved oxygen and fish kills in ponds
- Field experience of Aquiflor, aquatic garden centre in Nodebais (Belgium) for over 30 years
- Questions received by the Foudebassin.com customer service team
Last updated: 27 September 2026.
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