Falconut I've run into the same problem here and it seems to be semi-seasonal. It's my belief that it has something to do with possibly our ozone levels (at least in part).
Even after switching over to RO/DI water after using plain tap water for many years I still have ocassional issues with it.
Also the idea that Nitrates are the cause is subjective at best. I know many people who have 0 Nitrates including myself and still wind up getting cyano once or twice a year.
My best cure so far was given to me by a friend in our local reef club. Since she's doing much of her studies on algaes (even though cyano isn't a true algae) I thought she'd be the best source to talk about this ongoing seasonal problem.
Her idea was to actually slowly ramp up my feedings to the point where the nitrogen cycle restarts itself. Oddy enough, it's the only thing that works. I've tried Chemi Clean and other products with minimal to no beneficial results.
I increased my feedings over the course of a week (when it appears) as follows:
Day 1 - two, one teaspoon size feedings
Day 2 - two, one teaspoon size feedings
Day 3 - three, one teaspoon size feedings
Day 4 - three, one teaspoon size feedings
Day 5 - two, one teaspoon size feedings
Day 6 - two, one teaspoon size feedings
Day 7 - my normal one teaspoon size daily feeding
The amounts are spread out over the day using one teaspoon per feeding. Everytime I've done this it's worked and by the end of the week it's gone or almost totally gone. Within a few days it's erradicated.
The measurements are used for my 90 gallon reef tank so it should work in your tank as well (ideally).
All I can say at this point is there's more to it than simply Nitrates as everyone keeps theorizing.
I've also tried 5 different species of snails which I consider to be the best clean-up crews by far. They impact by them of controling it is minimal if at all most times. I've since gotten an urchin which is much less discriminate of the type of algae or even dinoflagelates. I'm waiting to see how he/she does.
So far it's very slow but also VERY effective at what it does remove
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