I know it may be an expense but I have had luck keeping algae down by using colored water lines. Mine are blue, and the reservoir is a series of 55gallon blue drums. I was lucky to get the box of blue tubing for free from someone...Maybe there is a way you can block light to the system without having to replace everything? Tape on the lines, and paint the reservoir if needed?
Idk how or why, but water sometimes stops flowing in random places and I can't find it if I can't see the water in them. Triggering the nip or cup to let air through doesn't work. Have to detach it and then detach filled line further up and blow it through, then reattach and see if it's working again.
But could probably paint most of the lines, leaving gaps around attachment points.
Oh! I bought heavier duty 3 day connectors, they're all starting to crack and leak. So, if the cups work out, I'll be buying the expensive metal connectors next.