My suggestion, as mentioned above, would be MarinePure’s Biofilter media, which you can check out on Amazon.
This bio media is far more effective at removing nitrates as they are porous in comparison to plastic. They have been specifically developed for saltwater aquariums with one sphere having the same surface area as 1350 bio balls made of plastic!
Not only can they can be used in sumps, but canister filters and wet/dry filters too. It is worthwhile noting though that as higher nitrification occurs when there is more oxygen introduced into the water, it would be more beneficial to use the spheres in, for instance, a trickle filter as opposed to simply sitting in the sump.
I would definitely recommend using this particular bio media over plastic bio balls.