This discussion has been going on since power filters were invented. I ran tanks successfully for many years with no filters (just air stones for a bit of water circulation, and occassional water changes), by having heavy planting and not too many fish. When power filters came along, everyone wanted to pack in lots of fish, heavily planted tanks became less fashionable, fishfoods got richer and richer (great for the fish, bad for waste products), and shops started selling huge quantities of garbage for 'cleaning' tanks and 'balancing' water conditions. The argument about gravel bacteria vs filter bacteria is not the right one. The answer is simple; if you want high stocking levels, accept that you'll need extended surface area for filtering (biological and mechanical filters), will have to syphon out surplus mulm, and may even have to add chemicals (yuk). But that seems all too hard to me. I prefer to spread the fish around more tanks, and keep my workload down to the odd water change!