"Green Water" (Euglæna) and "Infusoria" The aquarist's "green water" is commonly described as a dense bloom of free-floating single-cell algae. Many of these so-called algae are in fact species of Euglæna ("you-GLEEN-a"), a genus of photosynthesizing protists, and its close relations (hence "euglenoids"). Algae and euglenoids are among the simplest photosynthesizing organisms, commonly assessed as though they were a single family group. Together with bacteria they form the base of the food pyramid.

For very small fry, like larval labyrinth fishes, green water is an indispensible first food, yet sometimes the cultures come and go unreliably for me. "The surest cure for green water," I repeat glumly, "is day-old gourami fry." A drawback of apartment life is that west-facing windowsills are no substitute for a greenhouse or a garden pond--— or even a sunny south-facing porch. For rank beginners of course, green water seems to come without being called for. Carbonates in the water seem to help. With my soft acidic water, I need to spread a thin layer of crushed coral on the bottom of a green water culture, to provide the alkaline conditions (>pH7.0) that are required. A pinch of potassium chloride ("Nu-Salt") and a couple of drops of Fleet enema (phosphate) may help. But soon my green water is invaded by rotifers, ciliates and copepods; they graze on the Euglena like Goldilocks, til they're all gone, and the culture clears.

A pure culture of Euglena can be mailed to you from one of the live food culture sources in the last page of this folder. An uncontaminated culture of Euglena is quite beautiful: intensely-saturated emerald green, and as clear as the gem, too. I've never been able to maintain its purity: as my "green water" culture gets contaminated with rotifers and other protozoans it becomes progressively cloudier. Unless you can provide sterile lab conditions, the trick is to keep reculturing Euglena, in conditions so favorable for photosynthesizing that the grazers can't keep up. I can't provide enough natural sunlight to achieve this; my euglenoids won't be able to keep dividing faster than the protozoans can feed on them, and the culture will clear as magically as the "green water" in my first aquarium. And for just the same reason. And probably just when I needed it most, too. At this late stage in a green water culture, it's sometimes useful to add a batch of fish fry that are a little too large to be grazing directly on the algae and euglenoids, with the thought that they will prey upon the rotifers and ciliates that might otherwise suppress the algae. But it's a stop-gap measure at best; your "green water" culture is developing into an "infusoria" culture...