You are most kind, RM. We haven't solve your problem yet but it is a challenging one.

However, I have found something very interesting. Have you heard of the Redfield ratio? I guess not.

Well, apparently, some researchers did some work on the likelihood of getting the blue-green algae vs the green algae based on the amount of nitrAte and phosphate in an aquarium. There appears to be a certain ratio of this two elements whereby no algae will grow. The interesting thing is that it doesn't mean zero nitrAte or zero phosphate but a ratio of 16:1 of both.

What this means in practical terms is that instead of madly trying to reduced everything to zero, you find out what you nitrAte and phsophate levels are and then add or subtract one or the other until you arrive at the white zone whereby there is least chance of algal growth.

E.g. if your nitrAte is at 15ppm, you will need phosphate at around 0.75ppm to balance it. If your phosphate is 01.ppm than you will get lots of green algae!
I know it sounds radical and I don't expect you to rush out there and start adding fertilisers but it is an interesting concept. How about getting a phosphate test kit and do some tests and see if the result lines up with the table. Maybe you can try experimenting in a container of tank water while approximating the tank condition.
BTW, this is the original article;
http://www.xs4all.nl/~buddendo/aquarium/redfield_eng.htmThe author is Dutch so the English is a bit of a mixed bag but you can make sense of it.

Betty, what think you?