I have a terrible problem with hair algae

This hair alage in my experience will come from bad water. Are you sure your using a quality ro unit?

Yes on RO water and Kalk. From my reading in Delbeek and Sprung, it seems to be Bryopsis, which they categorize as a hair algae. I have tried their suggested solutions, but to no avail.

This type of algae really only occurs after bad neglect, high iron in the water or other contamminants in the water. Next inproper lighting will also contribute.

Cures - check your water purification and lighting. Ive seen this happen and its because of the water.

What make and model ro unit are you using? Are you sure the internal resin isnt spent?