a little more info could be helpful - what are your nitrates and nitrites?
Is your tank newly setup? In that case, you might just let the tank cycle a bit longer and the ammonia should drop naturally. When you see a zero ammonia reading, but you are recording some nitrates and nitrites, its time to do a 50%ish water change.
You can do a water change - that should help bring it down too. If you tank is setup, its possibly that you don't have enough surface area for anaerobic bacteria (I think I'm getting this terminology correct) and you could maybe use some more live rock. However, I believe this is unlikely because between the surface area of the sandbed, current live rock, and any filtration media, you ought to have plenty of bacteria.