I have used Purple up in my acrylic aquarium and no it will not stain it - purple up is basically just a calcium mixture. I'm not sure if it works or not but just remember that you have to have coralline algea in the first place in order for it to propogate and spread. If you don't have any on your rocks you may have to seed your live rock by either scraping some coralline off of someone's elses tank (razor blades take it off tank walls really well) or get a piece of rock with coralline on it to seed your tank.
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