What species of jawfish? Some jawfish get rather large. As such, they may not only be a threat to your seahorses, they would need a substantial sand bed, the addition of which is likely to upset your tank at least temporarily with sedement alone. Due to the small gill pores of seahorses, I assume you would be moving the seahorses out of the tank if you planned on adding a sandbed to their tank? Otherwise, I think the cloudy water could upset their respiration and stress them.