Ok so I need to purchase a GFO and then go from there.Doubt the salt has any impact. It’s as good as any salt out their.
If you think it adds to your phosphate, just mix a batch to 1.026 and test.
According to your posts, the only parameter out of range phosphate. GFO is a filter media, Rowaphos for example, run in a reactor, or just a media bag in flow.
Phosphate will bind to this media and over time, lowers phosphate…..your looking at say 0.03ppm to .1ppm.
Id work getting that number in check first before moving onwards to other things.