I had green water in one of my tank also.... What I did is complete blackout and 10% water change every other day.... took about 6-8 days to go away. On the 10th day water was crystal clear.... alot of plants kinda turn yellow but it'll bounce back within a week or so. Alot of ppl say to let it ride out but then you will be looking at green water for at least a month.
FazTeAoMar, I recently setup my 1st planted aquarium, about 5 days ago and now I'm having issue with Green Water as well :-| I don't think we have Pistia stratiotes here in New Zealand as you mentioned in your post, that seems to fix your green water issue. Can water sprite help with this issue as well? Just wondering what other alternative plants that are available here in NZ I can use LOL