+1 for vinegar.
I use along stick with microfibre cloth tied to the end.
You mentioned that you want to paint it so personally I’d bite the bullet get some totes for the fish and clean it well and then paint it with a few coats of Rustoleum oil base paint. Otherwise a spray bottle of vinegar or CLR with a long handle algae razor scraper. I have dozens of tanks and don’t have any streaks on the rear of any of my tanks, are you using glass tops under your canopy?