I would get your nitrates tested.
Phosphates aren't going to hurt a coral...or anything really. That's a misconception. Phosphates
might help algae grow, but that's a whole other topic.
If phosphates aren't being used up (ie. coral growth, algae growth, et al) then there's a pretty good chance of there being another limiting nutrient at work.
Nitrogen is the most likely, and nitrates are easy to test for.
I suspect you'll get a number that's effectively zero when you test, but get it tested to be sure. Any LFS should be able to test it for you, but NO3 kits aren't expensive.
Anything else interesting about the tank or the way it was started, etc?