I am using tap - but as I said, I've tested both that and the tank water and there is no readable nitrate or phosphate. No copper either. Silicate can be an issue as I have not tested for that.
In the year that I've been using tap water, I have only started getting the algae over the past few weeks. So though I do cede the fact that RO/DI would (and will) be a very wise choice, I do have my reservations in blaming the tap water for the issue on the whole.