What do your nitrates get to right before you do water changes? You've got some pretty big fish in there.
Extra filters do nothing to help get rid of nitrates, and with the fish you've got, the
nitrate levels are probably getting pretty high. The difficulty of adding more fish (aside from swimming space, and I know nothing about cichlid behavior) is keeping the nitrates from spiking too high between water changes.
This isn't to say that an extra filter is a bad idea. You can never have too many filters running, as long as you don't turn your tank into a whirlpool. It's just that there's a limit to what they can do.