I use api kh as well. I never had a problem with it. Purple up is snake oil, really not recommended for tanks that have stony coral. The best way to get coralline algae is to keep your big 3 (alk mag and calcium) in line. I bet if you brought your calcium up a bit to 450-500 you'll get it to grow. I wouldn't do anymore water changes. just let the alk drop on it's own.
Although, I never understood why anyone goes out of their way to promote the growth of coralline algae in their tank. True, it's a sign that you're doing something right but when you have to scrape that stuff off your glass on a weekly basis you grow to really despise it.