Most do not has tops on there tanks. Something to do with gas exchange. I'm not 100% sure why but I do no most do not and some even suggest it.
I have the door on my bio cube 29 open when I'm on and cracked when I'm not.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley