Okay, so I accidentally overfertilised one of my tanks. With nutrients AND sugar.
(I use sugar to increase CO2. More of an experiment than anything.)
I woke up the next morning to see the water bright green, the fish gasping at the surface, and a bunch of dead guppies.
Luckily when I went over, I noticed a few guppies had survived.
I added an airstone. (Ok, really just hose coming off a pump. Couldn't find my stone and this was an emergency!)
Changed 80% of the water.
Scooped out the dead fish.
Added carbon to the filter.
The fish went back to their normal behavior- the survivors ahd no signs of stress. Unfortunately, the water was still green. Not as green as before the waterchange, but quite green. I left it be, and monitored the fish's health. There was a trace of ammonia5 minutes after the water change, but oddly, later that night, the ammonia was gone. The air pump was tied into the lighting control, so it only came on during the day. I didn't do this on purpose, I just ran out of outlets. (only one outlet and two 10 gallon tanks... With the air pump, that's 7 plugs.)
The next day, water was still green maybe even more green. And the next.
I was getting worried that the plants would be in trouble, with such little light and no added nutrients.
The next day, the water was crystal clear. out of the blue. Not gradual, but overnight.
Just thought my technique might help others if they have greenwater.