The mobile creatures that make up greenwater, as you might have noticed in Immediate Help, are NOT algae. Usually they are classified in their own kingdom and called protists. (US biology Textbooks will speak of the kingdom Protoctista. UK texts call them Protista.) There evidently have been several proposed schemes for sorting out living things into different kingdoms. http://en.wikipedia.org/wiki/Kingdom_(biology)

A lot of commentators and books still say greenwater is algae, but too many of them also recommend floating new fish in their bag for 20 minutes and emptying everything into the tank w/o quarantine. So they may be 50 or 100 years out dated. ;)

There are a surprising number of creatures with chloroplasts in them. Among others in and on reefs, some corals and anemones have chlorophyll. I think some freshwater hydras do too.

"Greenwater (take a 10X or 30X kiddie microscope and check it out) is a mixed bunch of stuff, often protists like Euglena and Paramecium. They tend to feed on even smaller zooplankton and phytoplankton, but the green in the water is at least in part because even though they move around, they also have chlorophyll cells which show green..." when they are numerous enough. They are virtually always in aquariums. http://www.guppylog.com/story/2003/8/3/14145/91568

About 4.5 years ago Scott Lockwood was doing militant water changes and the water (thriving upon the new minerals and cleaner water) got even greener. Had you mentioned that the cloud was green, you would have gotten different advice.

As for Scott, he got rid of it. I don't think that he even had to cut down the light by either just not leaving the aquarium light on so long and/or  by pulling the shade on the window which let in sunlight in the afternoon. He did go to work with a gravel vacuum and found the spot lush with nutritious mulm (organic muck on the bottom) and took that out. The green water starved for lack of  nutrients.

Your treatment probably poisoned those tiny protists and then there was a coagulant in that solution, which bunched the dying or dead protists into larger clumps and they dropped to the bottom. (We've talked about flocculation before.)

And sometimes greenwater will just crash on its own and fall to the bottom of the tank. Both when there are crashes (ironically maybe because of not enough water changes) or when it is treated as you have treated it, then you want to vac that off of the bottom of the aquarium and get much of it out before it rots too much.

In fact, if one wants to spend a lot of money, they can get a UV sterilizer and run the water through it. An article on somehow using ultrasound to kill those members of the protist kingdom also is floating around. And I would still "sweep the floor." :)

Companies making pond and aquarium products usually use less dangerous poisons than in the past, but you also want to dilute that so that it causes your fish no harm.

You might look to see if sun light is hitting the tank at sometime in the day. Sun angles are so different in the winter that sometimes we are surprised that the tank is getting so much. Simply pull the shade before going to school or work.

In The Swim Algae Clear is made by a company in West Chicago, about 35 miles west of Chicago and maybe 55 miles from me. Their web site doesn't say what is in their product. I suppose I could call them up and inquire, but vacuuming your tank is probably sufficient.

20-30 years ago, Simazine was used a lot in algaecides. It is listed as a General Use Pesticide - or abbreviated GUP! An article by a guy in FAMA (Fresh Water and Marine Aquarium Magazine) demonstrated how if it was left in his cichlid tank, a number of the fry would be pretty badly deformed. Sadly that stuff is still used some, but there has been some effort to rid of products like that in the aquarium business.