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Phytoplankton is composed mainly of microscopic free floating or suspended algae and is generally referred to as greenwater by aquarium hobbyists. The green colour is the result of chlorophyll produced by algae, as phytoplankton is usually rich in green algae. However, it also includes diatoms, blue-green algae, and flagellates.
Phytoplankton contain proteins, starches, fatty acids and oils and are an important part of the food chain in fresh water environments. Phytoplankton also provide vital bio-pigments that are required for the development of proper colouration for the rainbowfish fry. An adequate supply of nutrients, including carbon dioxide, enables phytoplankton to transform the light energy of the sun into energy-rich chemical components through photosynthesis. As such, it is an excellent primary food source for newly hatched rainbowfish larvae.
Phytoplankton also serves as food for zooplankton, which in turn are fed upon by the fish larvae. Some have a symbiotic existence with lower animals such as hydra, which has a symbiotic association with chlorella algae.
The majority of phytoplankton is made up of holoplankton, organisms that spend most of their life cycle in the planktonic community. However, many phytoplankton species are capable of producing resting spores, which can to be found in deeper water or in the bottom sediment of fresh water habitats. These 'resting stage' spores are generally what cause the algal blooms that we often see in freshwater environments.
Most phytoplankton is very small, often-minute organisms ranging in size from 0.2 to 2 mm in length and drift with water currents. Like terrestrial plants, phytoplankton use carbon dioxide, release oxygen, and converts minerals to a form animals can use. In fresh water, large numbers of green algae often colour lakes, rivers and ponds. Some blue-green algae may be toxic or affect the taste of drinking water. Although, it is generally accepted that blue-green algae are actually bacteria and not algae.
Cultivation of phytoplankton for feeding rainbowfish fry is very simple. All that is required is water, light and nutrients, and if the culture is well managed will produce healthy juveniles. Natural sunlight is best, if not mandatory, as without strong light the phytoplankton will simply not grow. Perhaps the simplest way is just to place some old aquarium water from one of your waterchanges into a suitable container and store it outdoors. When it turns green you can then collect a portion of the culture each day to feed to your fishes. Generally, sufficient green water is added so that the tank will not be completely cleared in 24 hours.
If on the other hand you want to culture your phytoplankton indoors, then the best way is to use a bare 50-litre aquarium placed near a window that receives several hours of natural sunlight each day. Fill the tank with old aquarium water and it should turn green within a few days. Mechanical circulation can be used to keep the algal cells in suspension, thus exposing as much surface area as possible to the light for photosynthesis, while making sure they don't remain in the same spot too long for photo inhibition to become a factor. Cultures are generally maintained at a temperature range of 20-24°C, pH 7.5-8.5. High-density cultures may require a pH buffer to prevent pH drift.
You could even set up a bank of 50 litre aquariums as raising tanks for rainbowfishes and simply place the spawning mops containing eggs into the greenwater tanks for hatching. The culture needs to be very thick whereby; the bottom of the tank cannot be seen through the greenwater when looked at from above. When the eggs hatch the fry will survive and grow without any additional help from you. As the fry grow you can start feeding brine shrimp nauplii and other foods until they are large enough to be moved to a more suitable environment. As the fry develop you may need to add some form of filtration.
Phytoplankton also play a role in the maintenance of proper water quality through their ability to cycle nutrients (such as nitrates, phosphates, and potassium, and metals such as iron, copper, manganese, zinc, and molybdenum). Phytoplankton are also able to utilise the metabolites produced by the young larvae (ammonia, urea), converting them into nontoxic forms, and also play an important role in regulating the pH of the culture container through their removal of excess carbon dioxide.
The only problem you are like to face raising rainbowfishes in phytoplankton tanks is that the culture can suddenly crash, dying off as suddenly as it appeared. The phytoplankton simply consumes all of the nutrients in the water and then collapses. It's important to vacuum out all of the dead algae from the bottom of the tank as soon as possible. Otherwise, this organic material will consume substantial amounts of dissolved oxygen from the water as it decays and in doing so can kill all the fry. Therefore, careful observation of the tank is required at all times.
Most aquarium related cultures of this type are generally referred to as Infusoria. The term infusoria is a collective name for many microorganisms and can include certain algae, bacteria, protozoans, desmids, rotifers, paramecia and a host of other small organisms. Unless you are maintaining pure cultures in sterile conditions, your culture will have both phytoplankton and zooplankton in greater or lesser degree. Cultures maintained indoors without sufficient light will probably contain more zooplankton than phytoplankton but outdoors the situation is usually reversed.
However, a pure algae culture may provide a better quality nutritious diet, which can accelerate growth and increase larval survival. Pure algae cultures are usually required to raise other live foods such as daphnia, rotifers and brine shrimp.
Production of pure algae cultures is accomplished by providing a favourable environment for the species being cultured, and follows six basic steps:
- sterilise the culture water
- add nutrient enrichment
- inoculate (or start) the new culture from a pure algae strain
- make microscopic observations and cell counts
- harvest algae and feed to larvae
- maintain stock cultures
Algal culture begins with a pure stock or starter culture of the algal species desired. These can be obtained from a number of sources. Commercial biological supply houses often sell algal cultures, but the species may not be suitable for raising rainbowfish larvae. Perhaps the most common method of obtaining algae is to visit a fellow hobbyist or commercial hatchery, since many hatchery operators will be glad to provide starter cultures to other culturists.
Once obtained, starter cultures (usually transported in test tubes) are used to inoculate several new cultures. Some of these are kept as stocks for when an old culture dies, is harvested, or otherwise lost and must be restarted. The rest are used to inoculate progressively larger vessels until there is enough culture to start mass production tanks. The culturist must supply light, aeration, relatively stable temperature control, and sterile water with nutrients (media) to produce successful algal cultures. By avoiding major contamination from unwanted algal species and microscopic predators, a continuous, dependable supply of high quality algae will be available.
However, pure cultures can become contamination by unwanted algae, bacteria, and predatory protozoans, and is a problem that cannot always be controlled successfully. Cultures are easily contaminated from non-sterile containers or splashed water from buckets, hoses, or hands.
© Copyright Adrian R. Tappin
Updated July, 2005.
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