Fish of course ingest phosphates with their food, and need some phosphates. I wonder though what a detrimentally high level of phosphates might be though, as I assume that even with fish there is an upper level to that. Interesting to read that bacterial processes are necessary for converting organophosphates to inorganic phosphates, so some degree of biofiltration is necessary, even if you were trying to rely mainly upon plants to absorb phosphates.