You can pretty much solve the nutrient load with healthy macroalgae grow-out. Our 40g SH tank is 0ppm across the board. Low flow, and any other organisms kept with them should be "Seahorse friendly". Most snails and tiny hermits are fine, but Arrow Crabs, Emeralds, Sallys, etc. would probably be a bad choice. I wouldn't keep reef fishes with them, but some Pipes might work (keeping in mind the Glugea problem with H. erectus). Very mild, low-competition reef fishes such as Mandarins are also viable options, but they're really best suited to a species tank.