I've gone above the "max" stocking level in a tank. The one thing you need to realize is that it takes more work. You'll be doing more water changes, changing (or cleaning) filter media more often and performing water test more frequently. If I know that I'm planning on overstocking, I usually takes steps to help alleviate any concerns. I will add a bottom feeder and an algae eater to help clean the tank. I'll also add plants to help filter out some of the toxic chemicals and add oxygen to the water. I will say this: do not overstock a tank that hasn't been established for a while! And never add all your fish all at once, add slowly. Also, you really need to consider the temperaments of the fish involved. You may be able to handle to waste and water quality but end up creating stress simply because the fish doesn't have enough physical room. Now I know there will be a lot of people that will disagree with me, but I haven't had a real problem yet and I think a bio-diverse tank more closely resembles nature.