Algae in the medaka pond are rarely just a matter of cleaning. An Australian research project built on 16 years of data shows that the composition of algae can be predicted from water level, salinity, nutrients and temperature. Whoever has that list in order knows when algae in the medaka pond will take over, instead of finding out in July.

The study appeared in Continental Shelf Research and was carried out by researchers from Flinders University, with Daniel Chilton as first author. The work was highlighted on 26 October 2026 by Phys.org. The full publication is available from Elsevier.

What the Coorong study followed for sixteen years

The Coorong is a shallow coastal lagoon in South Australia, designated a Ramsar site in 1971. The lagoon runs from fresh water at the mouth of the Murray to brine in the southern arms. That makes it a natural laboratory: the same water, four climate zones in one system.

The researchers sampled phytoplankton across that entire gradient and set their own measurements beside 16 years of historical data. That period covers both extreme drought and floods, which separates the influence of water level and flow from the accidents of individual years. What they measured:

• the species and numbers of phytoplankton per sampling point;
• the salinity and water level at the time of sampling;
• the available nutrients, nitrogen and phosphate in particular;
• the water temperature and the season.

Salinity and flow steer the composition of algae

The outcome is not a list of species but a pattern. In the fresh and slightly brackish parts the team found the greatest species richness, with a clear dominance of cyanobacteria and green algae. In the extremely salty parts the number of species fell and the picture shifted to diatoms and dinoflagellates, in roughly equal proportions.

More important for practice is the combination that triggers a bloom. Plenty of nutrients at low salinity encourage cyanobacteria. Sustained drought with rising salinity encourages the harmful dinoflagellates. Of the species found, more than 80 can produce toxins, although the concentrations measured in the Coorong remain low for now.

Why a still pond resembles a lagoon

The Coorong is not a garden pond, and the study says nothing about medaka. The resemblance lies in the drivers, not in the scale. A pond without flow has exactly the two properties that determined the composition of algae in the lagoon: nutrients stay in and the water is not refreshed. Add summer warmth and the order of events becomes predictable.

In a pond, nutrients mostly come from outside: food that is not eaten, leaves that fall into the water, dying water plants. Salinity plays a part as soon as you add salt or the water evaporates during a dry spell. Water level and temperature follow the weather. Whoever keeps track of those four factors does not have to wait for the water to turn green to know that it will. Our earlier piece on algae in the medaka pond looks at control in more detail.

Spotting cyanobacteria before the water closes over

Green water is not automatically a problem. Fry thrive on fine green algae as a food source, as we described earlier with green water when raising fry. Cyanobacteria are a different story. They form a blue-green layer that keeps floating on the water, smell like a closed aquarium and care little about a few days of shade.

The risks are twofold. During the day algae produce oxygen, but at night they consume it, and a dense bloom can pull the oxygen level below the limit for fish by morning. On top of that, cyanotoxins can burden the gills and the liver. In young fish that happens faster than in adults, and the first signs are often more withdrawn behaviour and gulping at the surface.

A weekly schedule for clear water in the medaka pond

A fixed routine works better than intervening afterwards. The order below costs fifteen minutes a week and tackles the factors the study identifies as decisive. The order is not accidental: first the source of the nutrients, then the water, then the check. That is how you stay ahead of algae in the medaka pond.

1. Remove leaves, dead plant material and visible leftover food with a net.
2. Feed in two portions that are gone within a few minutes, and nothing after that.
3. Check the temperature and write it down, so a warm week does not come as a surprise and your medaka stay healthy.
4. Change part of the water during sustained heat, instead of all at once.
5. Look at the water against the light: floating layers and sharp smells are the earliest signal.
6. Leave oxygen-releasing water plants at the surface and do not let rampant plants rot.
7. Note what changes after each step. Without notes every summer becomes guesswork again.

Food is the biggest source of nutrients you control yourself. Our medaka fish food is made to be fully taken up in small portions, but even with good food the rule holds: what stays on the bottom feeds the algae, not the fish. For raising young fish the dosage is even more critical, because there you feed every day.

Anyone who wants to feed less but still supplement can use daphnia . Water fleas filter phytoplankton out of the water and are food at the same time. It is not a solution for a pond that has already closed over, but it is a way to slow an incipient bloom without chemicals.

What not to do when the water turns green

The urge is to change everything at once when the water turns green, even when it is algae in the medaka pond. That makes the cause untraceable and burdens the fish unnecessarily. These interventions rarely address the source:

• changing all the water at once, so temperature and water values flip within an hour;
• emptying the pond and starting over, which removes the bacterial film that kept the water stable;
• raising the food to make the fish stronger during a bloom;
• adding a product without removing the source of nutrients, after which the bloom returns;
• covering the pond and forgetting the aeration.

During a sustained bloom it is wiser to check the fish for stress and skin problems. Our guide on medaka diseases describes the signs that go with poor water quality, and the piece on caring for medaka sets out care per season. For anyone just starting with breeding medaka the summer is the period when water quality makes the difference between losses and a full tub.

FAQ about algae in the medaka pond and water quality

Is green water dangerous for young medaka?

Fine green algae are food for fry and usually not a problem. It becomes dangerous only with a dense bloom that demands oxygen at night, or with a blue-green layer that keeps floating on the water. As long as you see the fish gulping at the surface, something is wrong.

Does feeding daphnia help against algae in the medaka pond?

Water fleas filter algae out of the water, but they only work as long as there is no oxygen shortage and the bloom is not too far advanced. Put them in at the first signs, not when the water has already closed over.

Should I change water during an algae bloom or wait?

Partial changes are the safe route: they lower the nutrient concentration without flipping the water values at once. A full change does not solve the source and costs the bacterial film that kept the water stable.

What changes about algae pressure in autumn?

When temperature and light drop, growth declines. What remains is dying plant material releasing nutrients on the bottom. Autumn is therefore the moment to remove leaves and dead plant material, not to leave the pond alone.