5 reasons why green water is good for your fry

Als medaka fan en aquariaan of vijverliefhebber ken je het gevoel vast wel: je kijkt naar je bak of vijver en ziet dat het water een groene tint heeft gekregen. De eerste reactie is vaak paniek. Algen! Maar wat als ik je vertel dat dit groene water juist een van de beste dingen kan zijn die je jongbroed kan overkomen? In dit artikel leg ik uit waarom je die groene waas misschien niet meteen moet bestrijden, maar juist moet omarmen.

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Green water: pest or blessing for fry?

Many hobbyists see green water as a problem that has to be solved as quickly as possible. And honestly, in a mature aquarium with large fish that is understandable. But with fry the situation is very different. Green water consists mainly of microscopic algae particles, also called phytoplankton. These single-celled organisms drift through the water and together form a kind of living soup that creates an incredibly rich environment for newly hatched fish. The problem with fry is a classic dilemma: they are too small to eat normal food, but they need nutrients to grow and survive. This is exactly where green water offers the solution. Instead of dosing small portions of food three times a day and hoping the fry find it, green water provides a constant source of food that is always present. The young fish can graze literally all day without you having to do anything.

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How phytoplankton helps your young fish survive

Phytoplankton is the engine behind green water. These microscopic plants produce oxygen through photosynthesis and form the base of almost every food chain in water. For fry they are valuable for several reasons. First, they are small enough to be eaten by newly hatched fish, even species with a very small mouth. Second, they are constantly present in the water column, so young fish do not have to search for food. But phytoplankton does more than feed. It also stimulates the growth of rotifers, also called wheel animalcules. These small water animals feed on phytoplankton and in turn are an excellent food source for slightly larger fry. So you are essentially creating a mini ecosystem in your rearing tank. The food chain in green water looks like this:

1. Phytoplankton grows on light and nutrients
2. Rotifers eat the phytoplankton
3. Fry eat both phytoplankton and rotifers
4. Fry grow faster and have a higher survival chance

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Medaka Baby fish feed as a rescue when fry are starving

Starvation of fry is one of the most common causes of losses when raising fish. Many breeders only notice it when it is already too late: the fish become smaller instead of larger, become listless and eventually die. This often happens in the first week after hatching, when the yolk sac is empty and the fish have to find food themselves. If no suitable feed is present then, the chance of survival is small.Medaka Baby fish feed is a specific type of fish feed that is particularly suitable as a solution to this problem. It is a single-celled alga packed with nutrients. Below is an overview of the main benefits:

Nutrient	Benefit for fry
Proteins (up to 45%)	Supports rapid growth of muscles and organs
Chlorophyll	Promotes digestion
Omega-3 fatty acids	Important for brain development
Vitamins (B12, C, E)	Strengthens the immune system
Minerals	Supports bone formation and general health

This gives fry the best chance of a good start.

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Practical tips for using green water

Now that you know why green water is so valuable, the next step is how to use it properly. The aim is not to let your rearing tank be overgrown by algae, but a controlled amount of green water is exactly what you want. It is all about balance. Here are a few practical tips for using green water effectively:

• Use a separate rearing tank with plenty of light to grow green water before you need it
• Add the green water to the rearing tank as soon as the fry start swimming (free-swimming stage)
• Check the water values regularly, because algae can affect the pH
• Combine green water with infusoria for an even more varied food supply
• Limit the use to the first two to three weeks, after which you can switch to finely ground dry food or artemia nauplii

A common mistake is to see green water as a replacement for all other food. Think of it as a supplement and a safety net instead. It ensures that the fry always have something to eat, even at times when you are not at home or forget to feed.

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Read more

• Algae in the Medaka pond: prevention and control
• Top 5 Medaka feeds for fry in the Benelux
• Medaka breeding: from egg to adult ricefish

Frequently asked questions about green water and fry

Is green water harmful to my fish?

In controlled amounts green water is not harmful. It can even be beneficial for water quality because algae absorb CO2 and produce oxygen. With extreme algae blooms, however, the oxygen balance can be disturbed at night, so keep an eye on the amount.

How do I make green water myself?

Place a transparent container or bottle filled with aquarium water in a sunny spot or under a lamp. Add a small amount of nutrient (such as a drop of liquid plant fertiliser) and wait a few days. The water will turn green on its own through the growth of phytoplankton.

Which fish species benefit most from green water?

Especially protein-rich species such as cichlids, goldfish, carp and many tropical fish benefit strongly from green water as fry. It is also an excellent food source for shrimp and other small water animals.

How long should I use green water?

Green water is most valuable in the first two to three weeks of the fry's life. After that the fish are big enough to switch to other food sources such as artemia, microworm or fine dry food.

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Groen water is lang niet altijd de vijand die het lijkt te zijn. Voor jongbroed kan het juist het verschil maken tussen een hoge uitval en een succesvolle kweek. Door fytoplankton en algen zoals Chlorella bewust in te zetten, geef je je jonge vissen een constante voedselbron, stimuleer je de groei van radertjesdiertjes en verklein je de kans op verhongering aanzienlijk. De volgende keer dat je kweekbak een groene tint krijgt, hoef je dus niet meteen in paniek te raken. Misschien is de natuur je gewoon een handje aan het helpen.