This post we be a bit redundant to the others... Although better filtration and overall tank maintenance like routine partial water changes will allow a larger bio-load, this is similar to the fact that routine water changes are not all about removing nitrates. Fish need space to promote the best health and well being. Lets consider that we've taken creatures that in the wild, could live in vast rivers or lakes. Some may be content to live their entire lives in small pools and estuaries while others may roam for countless miles and miles.
We put them in these tiny glass cages. Sometimes letting the water get rank. Feed them processed man made food. Subject them to harsh, unnatural lighting so we can see them better...and sometimes crowd them with too many tank mates or other unfriendly territorial species that chase and hound them.
Some would say, 'but these are fish that were raised in tanks and never knew life in the wild'. This is true, but just means is our responsibility to do the very best for them.
I tend to like the diversity of the community tank. But then I saw a 55g with a couple of dozen neon tetras...and another 120g with a few dozen zebra danos - it was awesome watching these fish school.
Then I was at a pet store and saw two very large, very sad Otto's in a tank they could barely turn around in. How awful it must fell to be trapped in such a small space, with no hope of escape. (shame on them!)
The point, if there is one, to my rambling on and on here is that we need to be ever mindful that the more fish we put in the tank, the required maintenance goes up exponentially...and at at a point, the occupancy is full and the 'no vacancy' sign must go out.