Medaka Algae and Moss Control | How to Live Well with Green Water

When raising medaka, green moss can grow on the walls and bottom of the container, or the water can turn green, becoming "green water." Algae and moss are not necessarily harmful to medaka, but they can damage the aesthetic appearance of the aquarium and, if they grow excessively, can cause water quality deterioration. This article explains the types of algae and moss, their countermeasures, and how to utilize green water.

Types of Algae and Moss in Medaka Aquariums

There are four main types of algae and moss encountered when raising medaka. Green algae is thin green moss that attaches to the container walls and bottom—the most common type. It occurs naturally with light and nutrients, and in small amounts can even serve as food for medaka. Blue-green algae (Aomidoro) is thread-like green moss that forms fluffy clumps in the water. If left unchecked, it multiplies explosively and can trap medaka, preventing them from moving, so early treatment is necessary. Cyanobacteria (blue-green algae) forms a slimy, blue-green film with a distinctive odor. It's a sign of deteriorating water quality and should be promptly removed when found. Green water occurs when plant plankton (such as chlorella) proliferates in large quantities in the water, causing the entire water to turn green. Although it looks unpleasant, it offers many benefits for medaka aquaculture.

Utilizing Green Water

Green water is a "miracle water" with many advantages for medaka aquaculture. The plant plankton in the water serves as food for fry, dramatically improving survival rates of young medaka. For adult fish, it creates an environment with constantly drifting microscopic food, contributing to nutrition. Since plant plankton absorbs ammonia and nitrates in the water, it also has a water-purifying effect. Green water has been reported to enhance the body color of medaka, especially noticeably in red varieties. To create green water, place medaka aquarium water in a sunny spot and dissolve a small amount of food—it will turn green in a few days. However, excessively dense green water can cause oxygen depletion at night, so if it becomes so dark that the bottom is not visible, dilute it by adding fresh water.

Algae Prevention and Removal Methods

Algae prevention is fundamentally about controlling "light" and "nutrients." In environments exposed to direct sunlight for long periods, algae multiplies rapidly, so using sun screens or shade nets to block afternoon direct sunlight is effective. Excessive feeding increases nutrients in the water and can cause algae growth, so aim to feed an amount that will be consumed in 2-3 minutes. Green algae attached to container walls can be easily removed with a melamine sponge. Blue-green algae can be removed by wrapping it around a wooden stick or toothbrush. If treated while small, it takes little time, but if it has multiplied extensively, a reset (complete water change and container cleaning) may be necessary. Cyanobacteria can be removed by locally applying hydrogen peroxide solution or through three days of complete darkness. Regular water changes (once weekly, about one-third of the total volume) are also fundamental for algae prevention.