phosguard. low alkilinity

125 reef gallon tank with 30 gallon sump. I always had a phosphate problem. Testing it was always above 1.5 ppm. 3 months ago I had a bad Cyano outbreak so I bought and have been using phosguard. I got it down to between .6 and .7 PPM. I Been following the directions and change the phosguard...