this guy had the same problem i did with my phytoplanton crash
his steps were a lot more involved & documented where mine, all i can say is the end result Jon Olavs Akvarium
(home page) Plankton project June 2011
(page in question)
"One thing that is really frustrating for an aquarist is to be stuck. Basically to know that there is a problem, but simply not be able to find its cause. My cell concentration problem is like that. There are a number of possible causes to why cell concetrations of all types of cells grow to a certain point, and stop. For example high concentrations of bacteria that attack phytoplankton. Virus or some type of microscopic grazers are also a possibility. Or, most likely, something that I just don't have a clue about what is. Right now I am testing out the simplest hypthesis, namely that it is caused by nutrient limitation. I didn't think that could be the cause earlier because the two nutrients that I can measure; nitrate and phosphate, are present in sufficient concentrations during crashes. And the F/2 medium should be fairly balanced. I have also tried dosing more nutrients when I see reduction in cell concentrations, without seeing any changes. But I still think there is a possibility for nutrient limitation. My current dosing scheme should make it clear."
i have heard people cite "nutrient deficiency", ... this guy was dosing to ensure nutrient availability
(yes i know he's using saltwater vs. freshwater)
from my main tank, phytoplankton dies within days
from my greenwater bucket, i have ignored it for months, it's still thriving, nice and dark (tends to get yellowish if nutrients get too high, i have since only removed enough to ensure a nice dark green and added water to keep it from drying out)
mixing water between the two, the mixed water stays green
no one seems to really know why the greenwater would crash in the main tank other than citing "nutrient deficiency" yet the greenwater bucket is doing fine for months (i'm quite literally ignoring it other than topping up water)
from my own trial-and-error;
i stopped adding greenwater to the main tank for about 3 months or so, ... when i tried adding more after this time the greenwater stayed alive for ... about a week instead of the usual 3 days, ... and adding more greenwater after this was once again 3 days before the water was crystal clear
i am working on waiting 6 months before trying again,
the only differences i'm seeing is one (display tank) has several inches of fine gravel (never vacuumed) substrate with heavy plant growth
the bucket has detritus that has accumulated on the bottom (and i'm sure is beyond mineralized aside from whatever new is added due to dying phytoplankton, and the whole cycle repeats.