Phosguard might help in a limited capacity. It not the same as having all the water forced through it as if you had it in a filter sock or cup under a drain to the sump. It'll have some effect for sure just couldn't guess how much.
I would personally let them soak for roughly 2-3 days than 100% water change. Giving a little time let's water get deeper in them.
From one change to the next you should definitely see a decrease in po4. How much of a decrease will give you a fair guess of how many water changes it might take.
Also would help if you gave very breif rinse of rocks with ro and the brute. To remove anything that may have fell of during the soak before new water.
I'm honestly not even sure uf this needs to be saltwater or if you could just use rodi which would be easier and cheaper.
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Looks like rodi would work since your trying to stop them from. Leaching and not actively trying to cure or seed them.