Greenwater Culture
If you have a large breeding program you could set up a bank of 50 litre aquariums as raising tanks for rainbowfish larvae and simply place the spawning mops containing eggs into the phytoplankton tanks for hatching. The configuration of the system is simple and maintenance needs are minimal. The culture needs to be very thick whereby the bottom of the tank cannot be seen through the green water when looked at from above. When the eggs hatch the larvae 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.
Culture of phytoplankton indoors allows control over illumination, temperature and nutrient level. The most crucial part is getting the correct amount of light and nutrients. Strong light to simulate the outdoor brightness is required. The duration of artificial illumination should be a minimum 18 hours of light per day. Make sure the lamp’s output is “daylight”, (6500 Kelvin). A compact florescence lamp (55 watts, 7000 Kelvin) should give you sufficient light if the tank is not too large or deep (due to light penetration issue). Use the following rule: 0.7 watts per litre of water at a minimum of 6500 Kelvin full spectrum white light for 8 hours per day. The minimum wattage required is 55 watt. Lights can be purchased according to the size of the aquariums that are to be used for culturing.
Tanks are filled with dechlorinated water then fertilised and allowed to stand until the water turns green. Organic fertilisers (manure) are usually preferable to chemical fertilisers. Chemical fertilisers may be used and usually work better in earthen ponds than in tanks or tubs. Organic matter will produce bacterial microbes and detritus as well as phytoplankton as food for the larvae.
The fertiliser can be added to your culture in several ways. One is to soak the dry material in water until it breaks down into a thin mixture, then pour the resulting concoction into the tank, allowing it to slowly deteriorate. Another is to place the dry material in a mesh bag and suspend the bag inside the tank. Nylon stockings work well for this purpose. The use of a bag controls the organic matter and prevents suspended particles from being a problem. It also allows greater control of fertilisation. If fungus occurs on the bag containing the organic material it should be removed from the culture.
During the first 21 days of the larval culture period, water within the culture tank should be static with light aeration, just enough to gently keep the phytoplankton moving within the water column. Water exchange is not required with the greenwater technique and water only needs to be replaced due to losses from evaporation.
The presence of phytoplankton in the tank will maintain good water quality by taking-up the excess nutrients produced in the system. Phytoplankton helps the maintenance of proper water quality through their ability to cycle nutrients (such as nitrates and phosphates). Phytoplankton are also able to utilise the wastes produced by the young larvae (ammonia, urea), converting them into non-toxic forms, and also play an important role in regulating the pH of the water through their removal of excess carbon dioxide.
In a green-water system most of the nutrients are recycled back to the fish. With light aeration using an airstone, detritus, faeces and plankton can be kept in constant circulation. Nitrifying bacteria will colonise this floating “substrate” creating a “suspended growth treatment process”. These bacteria oxidise toxic ammonia nitrogen into relatively harmless nitrate and heterotrophic bacteria (bacteria that consume organic matter) proliferate. Rainbowfish larvae will feed on these and receive supplementary nutrition.
The suspended growth treatment process maintains adequate water quality for the fish while recycling waste nutrients into phytoplankton and bacteria that encourages continued algae and bacterial population growth, which further improves water quality. An added advantage of this process is the elimination of the need for a fixed-film biofilter.
Once the larvae are added the whole system can be managed according to the densities of phytoplankton and water quality. Water quality within the green-water system tank should be monitored on a regular basis, with water replacement performed if ammonia or nitrite levels approach 1-ppm levels. Tank bottom cleaning using narrow-gauge siphon tubes should be conducted on an as-needed basis to remove fungused eggs, dead larvae, etc.
Larval stocking density should approximate 20–30 animals per litres of culture water. Stocking densities higher than this will require more attention to water quality parameters and will likely result in higher larval mortality or wide size variation among juveniles. Stocking densities below 20 larvae per litre may result in better overall growth and survival. Larvae will grow more quickly and show fewer deformities at lower stocking densities than at higher densities. However, stocking density is dependent upon the number of fry required, and the quantity and quality of the cultured phytoplankton.
The only problem you are likely to face raising rainbowfishes in green-water system tanks is that there is the risk of the culture suddenly ‘crashing’ (failing), dying off as suddenly as it appeared. The phytoplankton simply consumes all of the nutrients in the water and then collapses and the larvae are left with nothing to eat. If this happens, it is important to vacuum out all of the dead algae from the bottom of the tank as soon as possible and start feeding an alternative diet. 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. The whole concept of the “green-water technique” is to mainly get the larvae through their early life stages until they can start feeding on larger foods, which is easier to provide.
Artificial Green-water
Mix a suspension of 1 level tablespoon spirulina powder per litre of distilled water and fed at a rate of about 10–50 ml per 20 litres - but only if the water has cleared from any previous feeding. The amount of the powder is not so important that you need exact measures. It should not be so much that it won’t go into solution and should be enough to make the mixture a dark green colour.