On the other hand, guppies are browsers. The new born fry will look for food in that stuff between feedings. By itself, it would be pretty thin soup, but it may be a useful dietary supplement.

Some aquarists have even isolated tattered or sick fish in greenwater as a hospital tank without adding medicines. One commentator suggested the greenwater had a certain antiseptic quality, maybe because these wee beasties were absorbing waste materials throught their photosynthesis.

One of the aquarium plant hobby publications claimed that nitrites and ammonia measurements were negligible in g.w. For being unsightly, it isn't much of a threat to the guppies unless it all dies suddenly.

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.

A lot of aquarists who want to raise very small mouthed fry or food cultures such as daphnia or something like rotifers, covet greenwater - which one can grow "all the time, except when one needs it."

Guppygirl has a very useful suggestion in limiting the light. Here is a dilemma for you. If it dies real fast, it will probably settle out. You want to get it siphoned out as soon as possible so that it doesn't rot, drawing oxygen out of the water.

Greenwater growth is stimulated by water changes, by the way. What works to limit or get rid of conventional algaes just stimulates greenwater to grow more. When I get a good greenwater tank going, I can remove 40-50% and it will replace itself in a day or two.

You may discover that this time of year  that the angle of the sun has "moved" north. Probably some time in the day - when you are at work - that green tank is getting some pretty good rays.

I believe that Sunlight is crucial to some cultures. My 20 gallon greenwater tank (used as a grow-out tank variously for livebearers, rainbows and killies) is reliable for 9-10 months a year. This time of the year, that tank goes pale. Fortunately (for the daphnia cultures) a rather crowded ten gallon tank in the northwest corner (with few plants to compete with the greenwater) catches the sun better and now looks like the Chicago River on St. Patrick's Day.

That might explain why only one of your tanks went green. It also suggests a safe solution.

A moderate limiting of the light might be to find out which window the sun could be shining through and cover it. As the greenwater gradually clears, pop a whole bunch of hornwort, Najas or water sprite in the tank to out compete the greenwater.

Bleaching the tank is a little bit like swatting a fly with a Buick. (Never use anything larger than a Subaru for fly swatting.)

There are also flocculants which cause items in suspension to settle out. I know they can be used in some cases in aquaria. Which flocculants and how they are used is out of my league. They also sound expensive.

You noted overfeeding as a possible cause. I think you are correct that it can be a factor, but if you have a few small snails in there on clean up duty, that may be better for the fry than underfeeding them.

Parttimer mentioned a product called Algae Fix. I am not familiar with that particular algacide, but a very sobering FAMA article several years ago analyzed several aquarium and pond "medicines" designed to get rid of algae. Many of them worked but had a chemical which seemed to accompany a striking number of birth defects among fry conceived and born in those tank.