Here's a shot of my 75 gallon tank yesterday morning. This tank gets 100% water change daily. I also wipe at least the bottom pane every time (yes, I'm obsessive). The fish get 2 feedings a day of beefheart. The rest of the fish, including some rams and corydoras, get Tetra-Bits and the like. One nice thing about barebottom tanks is that the amount of food you feed can be calibrated exactly. Everything was eaten and I know they can't eat more than this.
See how much fish poop is sitting on the bottom of the tank? All this is just one day's accumulation. With gravel and sand, you wouldn't see this stuff. A planted tank with substrate can't really ever get cleaned. Even if I were to vacuum the substrate every day, there would be no way to get rid of all of it.
See those stacked sponge filters in the background? They get squeezed once a week. When I squeeze do that, the water in the bucket turns black. Most aquarists think the tank is clean because the water is clear. In fact, the dirt is just hidden.