kent Marin phos sponge is safe for fresh and salt. as for dosage it doesn't really say but I would use it until phos test shows 0 or until the test start showing measurable phos levels. instructions state to use from 8hrs to 2 days use.

http://www.marinedepot.com/Kent_Mari...FMCHPR-vi.html

I have about half lb of high capacity gfo but at $36 a lb I won't give it away. you can have it for $10