The difference is in a normal tank when the fish produce waste its diluted and then at the end of the week the waters changed. In a heavily stocked tank the percentage of crap to water is alot higher, even if you change the water daily theres still a higher exsposure to crap.
Again, back to my comparisons... if one person craps in the corner of a hall, you are far less effected than if 10 or 20 people crapped in a corner or around the room aren't you? even if someone came in a scooped it up at the end of the day is that going to make you feel better?
My fish can happily live in a 300 L tank yet, they are in a 1100L tank with more than double the necessary filtration :roll: so you can't tell me all fish tanks are the same.