Green water, in aquaculture, is water intentionally cultivated to be rich in suspended, single-celled microalgae, primarily phytoplankton like Chlorella or Scenedesmus species. This dense, greenish suspension acts as a foundational food source for the aquatic food chain. Its main purpose is to serve as a highly nutritious diet for newly hatched fish fry, shrimp larvae, and live feed cultures such as Daphnia and rotifers. Cultivating green water offers a cost-effective alternative to commercial feed for early-stage aquatic life, while also contributing to stable water quality.
Necessary Equipment and Nutrient Sources
The foundation of a successful culture begins with selecting a clear, non-toxic container, such as a glass jar, clear plastic bucket, or a dedicated aquarium. Since light penetration is required for photosynthesis, opaque or dark containers are not suitable for initial cultivation. The water source should be dechlorinated tap water, aged aquarium water, or filtered natural water, as chlorine will actively kill the microalgae. The ideal temperature range for most freshwater algae species falls between 60 and 80 degrees Fahrenheit (15–27 degrees Celsius).
The microalgae require specific nutrients to multiply rapidly. A common and accessible nutrient source is a water-soluble, high-nitrogen general-purpose plant fertilizer. Nitrogen is needed for protein synthesis, while phosphorus is crucial for energy transfer and cell structure. Specialized aquaculture nutrient solutions, such as the F/2 medium or commercial green water kits, offer a more balanced and complete profile, including trace minerals that support optimal growth.
When using terrestrial plant fertilizers, it is important to choose a type that is free from algaecides or copper, as these elements are toxic to the microalgae. Another simple alternative is to use “manure tea” or water from an established fish tank, which is naturally rich in nitrogenous waste products. For a small-scale culture, a starting concentration of approximately 1.5 milliliters of liquid fertilizer per gallon of water is a typical starting point.
The Process of Algae Cultivation
Once the water and nutrients are prepared, the culture requires a starter inoculum, which is a small quantity of existing green water containing the live microalgae. This starter can be purchased from a specialized supplier or taken from a healthy, mature culture. The starter is mixed into the freshly fertilized water, which is then placed under appropriate lighting to initiate the photosynthetic growth cycle.
Algae thrive under light with a color temperature in the daylight spectrum, typically between 5000K and 6500K. An inexpensive LED bulb or strip light works effectively, and the light should be set on a cycle of 12 to 16 hours per day. Placing the container near a sunny window is also possible, but growth rates can be less consistent due to fluctuating natural light intensity. The microalgae consume carbon dioxide and utilize the light energy to convert the added nutrients into biomass, causing the water to turn progressively greener.
Maintaining constant movement is important to keep the algae cells suspended in the water column and ensure all cells receive equal access to light and nutrients. This is best achieved with gentle but continuous aeration using an air stone and a small air pump. If aeration is not used, the culture must be manually stirred multiple times a day to prevent the algae from settling at the bottom. Under ideal conditions, a new culture will typically transition from light green to a rich, opaque dark green within five to ten days.
Monitoring, Harvesting, and Usage
Monitoring the density of the green water is necessary to ensure the culture is healthy and ready for use. A simple visual test involves checking the opacity by attempting to view a white object through the water. When the culture reaches a point where the object is no longer visible at a depth of about 6 to 7 centimeters, the algae density is near its maximum and is ready for harvesting. If the water begins to lighten or turn yellowish-brown, it is a sign that the nutrients are depleted or the algae cells are beginning to die off.
Harvesting is accomplished by siphoning or pouring off a portion of the mature, dense green water for immediate use. When harvesting, it is important to leave a sufficient amount of the dense culture behind to act as the starter for the next batch. The harvested volume should then be replaced with fresh, dechlorinated water that has been mixed with a maintenance dose of nutrients. A common maintenance dose is about one-quarter of the initial fertilizer dose, applied every 10 to 15 days, or whenever a portion is harvested.
The green water is introduced directly into the rearing tanks of fish fry or live feed cultures like Daphnia. The microalgae provide a stable and easily digestible food source for these small organisms, promoting faster growth and improved health. Furthermore, the dense algae population helps to improve water quality by consuming excess ammonia and nitrates, and by releasing oxygen during the day. Maintaining a continuous supply involves rotating multiple culture containers, ensuring that one batch is always maturing while another is being harvested.