GFO is definitely much more cost effective than PhosGuard. What are you using to test phosphate? I would suggest you test it again to find out what you are at. I wouldn't use GFO is phophate is at 5 ppm, GFO will definitely shock the system. I would use GFO only on tanks already below 1 ppm.
If you use GFO start with just a small amount...like 1/4 the PhosGuard you're using.
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~ Mindy
SPS fanatic.
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