Last week I needed green water in order to do Lab 1-2 (Net Primary Productivity & Biological Oxygen Demand). It's not easy finding it in the middle of January, but on the day of the lab I drove to my most reliable source: a 1-acre pond in a small park heavily trafficked by waterfowl. To my dismay, the pond had been drained for cleaning! With little time to spare, I needed a substitute, so I used the filamentous algae that was invading my fish tank. I distributed the algae equally into light and dark flasks containing aquarium water. It served me so well that I used it for the following week as a substitute for Elodea in Lab 3-1 (Carbon Cycling Between Goldfish and Elodea). The advantage of using filamentous algae over aquatic plants like Elodea is that it is much more uniform, so you don't have to worry about removing discolored portions of the plant to maximize photosynthesis. About 3 grams of filamentous algae per 270 mL flask will increase dissolved oxygen about 2 ppm per half hour. This mass is obtained by simply pulling the algae out of the water and letting the water drip out for about 5-10 seconds (no squeezing!).
For the record, I am not sure if algae used was really Spirogyra because there were no working microscopes in the classroom, but this makes for a sexier posting title (especially for baby boomers who listened to pop music in the early 1980's).