What fish are in with your greenwater? If there are no fish, put some in there. It is a more efficient use of the tank and a bunch of fish poop is better, but safer feed for the greenwater than rotting flake food - which will feed more planaria than euglena, paramecium and the other protists which make up greenwater.

You want to set up your tank like it was a plant tank. Just forget to add plants. ;)

I have a Kelly-green 20-gallon set-up with a breeding colony of Ameca splendens, the butterfly goodied. They eat tons of veggie flakes and presumably produce lots of greenwater food. They love the current from the small power filter - without any filter medium. It is there just to move the greenwater around and create surface turbulence for the fish.

Extra chickletts, parasite-free gold fish (probably not feeders) or apple snails are all wonderful sources of poop for the greenwater. You don't see a lot of the fish or snails, but those tanks are also great grow-out tanks. They are surprisingly healthy systems (so long as the greenwater doesn't settle out or die off all at once.)

Have actually had a couple neon dwarf rainbowfish grow up in greenwater tanks. The parent, if fed, left them alone. That is why they could be called "the guppies of the rainbows."

I wouldn't want any filter element - it would remove the greenwater. Sponge filters or diatom filters would be the very worst that way.

Unlike with conventional algaes, the more you partially change the water, the better it thrives. I pulled  about 7 gallons of greenwater tonight out and ran it into the 40-gallon daphnia culture on the floor. A bucket and a 1 cm diameter siphon tube and the Ameca tank was soon full, verdant and flying again.

By tomorrow the greenwater will be as thick as it was today. The Daphnia are cheering.