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The control of red algae or any other algae all depends upon control of nutrients, mostly nitrate and phosphate. Unless you control these, you are going to have algae problems.
Just because your test kits reads zero or a very low value, don not think that there is no problem. Nitrate and phosphate can exist in and on the live rock itself.
I'm not a big fan of red slime removers. Yes, they do work, unless you fix the underlying problem, all you do is change the problem algae color from red to green or brown.
Basically in a nut shell if you have algae growing you have both a nitrogen source (ammonia/nitrite/nitrates) AND have phosphates in the tank. The problem with testing for it is that the algae WILL use it up as fast as it's being introduced into the tank. Since the algae is using it up quickly there isn't anything left to test for.
So if you have algae you have an excess nutrient problem.