waynecav let me explain why you shouldn't add another.
As the tank gets more loading it becomes more precarious.
So say some other factor comes along. Say the power goes off for a few hours. Or the temperature rises 2-3 degrees on a really hot day. Bingo! Like a house of cards the stress to the fish skyrockets. Someone gets sick - maybe the ammonia spikes, disaster. So yes you could put another fish in... But it may mean the tank crashes immediately, or down the road when otherwise it wouldn't.
I too feel you're at the limit now.