Make sure that your salinity is accurate. Best to use a calibrated refractometer to test it. The plastic hydrometers are horribly inaccurate. I have seen them out by .003.
If your Alk is low(< 7.3dKH) it can cause problems.
Phosphates will cause problems for corals if they are too high(typically >0.5mg/l and you will get problems, especially trying to adapt new corals).
As for Phosphate removal I would recommend Foz Down. I make and use Foz Down so I know it works and is safe. It is cheaper and easier to use than GFO or pellets.
Here is a link
http://www.oceanfreshaquarium.com/foz-down/
You might want to try adding some Iodine as well.
Water changes are one of the best ways to maintain a healthy environment for your animals.
Cheers,
Tim