Try a razor blade, be careful though. Id suggest putting a little oil on the edge of the blade to avoid scratching the glass, though that might still happen. diatomic organisms like algea are tough to get off if they have had time to accumulate on a surface. When they die, they continue to stick to the glass surface, and new liveing algea gover over top of them, so you get a tough calcification(sp?) effect that is a paint to remove. Once the algea dies and dries out, it will come off alot easier. rtaher than soaking it, let it dry real good and try scrapeing it off. And before adding fish, insure that the tank is cleaned and rinsed excessivly, those chemicles are insta death to fish.
I'm a guppy guy myself, angels are beautiful, but they will be quite a differant set of obsticals. Goodluck on your fish keeping, w/e those fish might be.