What lighting do you have on the tank, and how long do you leave it on for?
As dk said, a blackout will get rid of the algae, however in most scenarios it has to be for at least ten days. But, if you don't find the source and fix it from there it will keep coming back.
You describe the algae as really hard to scrub off, this is most likely spot algae and nothing will touch this as far as algae eaters. The easiest way to get it off the glass is to use a razor blade (do not use a razor blade if you have an acrylic tank).