I have discussed this before whether sea grass is a good nutrient export with some other people on other forums.... The problem is this it grows quickly and is very invasive as part of a tank, but I think if it is part of a habitat and not a method of removing nutrients than it should be fine. I think nutrients are probably best dealt with by much simpler and rapidly growing species. If you keep seagrasses, you should do so in their own right as a habitat. I would use fast-growing, non-invasive, non-toxic, easily harvested macroalgae, bacteria in the sand, or turf algae as biological nutrient removal.