I must have read the shell/phosphate issue on some carx thread and thought it was science, my bad as I know better, lol.I see this, or a variation thereof, said somewhat frequently. Do you have a study link? I have read that shells are approximately 95% calcium carbonate and 5% organic material .... which certainly would include some phosphates. But the same might be true for coral skeletons.
Either way, I removed any big shells I saw from the reborn, but did not go too crazy and my phosphate levels haven’t moved.
Not sure I would use all shells in a carx but they are not going to impact phosphate levels in any way, imo.