Jhale,
I have found, people who make their own Phyto/green water have tons of it and wind up throwing it away and giving it away. Some do sell it in 2 liter bottles, but I personally, don't think you'll need that volume unless you are culturung clams etc. Also, I have found, if I dose too much my skimmer goes berzerk - one time I lost a few gallons of water becaus emy skimmer went nuts.
I tried all kinds and personally liked the spray dried phyto from ESV - it stays in the water column longer and is absorbed by all animals that like it. My experience has been positive with this. Just my .02
House