of course you don't have to put a substrate in there! its called a bare-bottom aquarium. It can be a kindof cool industrial look. But the thing that might get in the way is that there is hardly any beneficial bacteria area then. I know beneficial bacteria like to chill in the substrate, so i guess you wouldnt have much of that. It also can be cumbersome to keep it clean, gravel and sand and whatnot keep it pretty hidden