Green Water Filtering

Dear Sean:

It’s good to hear that your greenwater tank for culturing microalgae (phytoplankton) and copepods (zooplankton) is working so well for you thus far. But I don’t think that connecting it to your established 10-gallon tank so it could function as a sort of refugium would be useful.

Even if you removed the sponge from your Aquaclear it would still tend to filter out the microalgae via the porous activated carbon, and the protein skimmer in the established tank would also kick into high gear as it attempted to remove the suspended microalgae and other organics via the foam fractionation process. Batch culture tanks normally do not have any form of biological filtration; typically they are just bare bottom tanks with mild aeration, good lighting, and nothing else, although the phytoplankton itself does a pretty good job of soaking up nitrogenous wastes.

It is a good idea to have two or more cultures of copepods and/or rotifers running at the same time, but I think your seasoned 10-gallon tank will serve you best if you just feed it with copepods and Artemia nauplii as needed, rather than attempting to culture the copepods in the tank itself.

Maintaining a greenwater nursery tank that houses a goldfish bowl style kriesel for the pelagic seahorse fry in it is a good thought and is certainly worth a try. Here is some additional information regarding such nurseries if you want to pursue that option:

The Greenwater “Starter” Nursery.

Basically, this system involves giving small numbers of handpicked fry a head start by raising them in a tank with a well-established greenwater culture for the crucial first week or two of their lives. A tank of greenwater is set up in a well-lit area and once the microalgae culture has taken off, it is seeded with copepods or rotifers. The microalgae acts as the filtration, utilizing nitrogenous wastes for growth. The idea is to provide a balanced system in microcosm with a self-sustaining food chain: the phytoplankton (microalgae culture) utilizes sunlight and nitrogenous wastes for growth and helps maintain water quality, while zooplankton (copepods or rotifers) feed on the microalgae and larger predators (seahorse fry) keep the ‘pod population in check. Additional greenwater and/or copepods or rotifers may be added periodically as needed to keep the nursery going.

In actual practice, it is difficult to for the home hobbyist to maintain the proper balance for any length of time before the greenwater culture or the population of copepods (or both) crashes. As a result, greenwater nurseries have limited applications and are useful primarily for the short-term rearing of small numbers of pelagic fry.

However, they can be quite helpful in giving pelagic seahorse fry a badly needed headstart. For example, once the hobbyist has culled a large brood of Hippocampus reidi fry down to a manageable number of the hardiest newborns, they can be introduced to a greenwater nursery where they can fatten up on tiny copepods or rotifers. This can help get the newborns through the first week or two of their lives until they are able to accept first-instar Artemia nauplii and can be transferred to a conventional nursery for further rearing.

The turbidity provided by the greenwater helps keep the phototactic fry evenly dispersed throughout the water column and away from the surface. Jorge Gomezjurado has been very successful rearing Hippocampus reidi and H. ingens fry at the National Aquarium in Baltimore using kreisel nurseries with the proper density of microalgae (i.e., greenwater). Jorge has found that turbidity is an important factor in the juvenile rearing environment for these species and he achieves the proper level of murkiness for optimum results by using algae (Nannochloropsis and Isochrysis) at a concentration of about 100 cells per ml (Bull and Mitchell 2002).

Best of luck with your culture tanks and the nurseries you are planning on testing, Sean! Here’s hoping your ponies cooperate and provide you with a healthy brood of fry very soon now.

Respectfully,
Pete Giwojna