Well, with plenum systems you add a screen 2 inches above the bottom to prevent burrowing critters from getting to the bottom layer. It is not very pretty but it works. The plenum system is not without its problems, I have excessive coralline algae growth, it grows like crazy on my glass, it sounds great at first until you have to start scraping it to see. I think it is a result of high calcium levels caused by the plenum. When I get to my next duty station I think I will add SPS when I have my tank set up again. Sharks would make a mess but they would still make a mess in a barebottom. Also in a plenum system you do not have a growth of anarobic bacteria, I think you would have to use mayo or something similar to get that to happen in a aquarium anyways, microaerophilic bacteria do grow plentifully in a plenum and that is what does all the reduction in wastes. I have heard that heavy skimming is required and oxygen levels can drop to dangerously low levels at night. This has not been observed by me because I have a fuge with cheato with 24/7 lighting and have a good skimmer on my tank. The plenum system is a BAD idea though if you like seeing stacks of live rock, I have grown partial to that in my fuge and nano and love seeing all the growth on it so will probably put a shelf in my next tank to keep the sand uncovered and get a large stack of live rock. The trick is though to make it not look tacky.