Closing off an open top?

What about an acrylic top that you can drill small holes in for air exchange?

 

several have said that will work, I personally don't trust it to move enough air for proper exchange without having to risk a fan close to the water.

 

im the other vote, i was going to say egg crate. but after reading the threads on the screen stuff. i changes my mind ;D . now i gotta figure out what to do with the sheet of egg crate sitting next to my tank.

 

That's how I've had my tank covered. It's pretty cheap and will definitely prevent jumpers. I don't trust the net to do that unless it is a very small grid. If you have a sump and skimmer, you really don't need to worry too much about the air flow above your DT. It's good to have and the air holes will allow gas exchange, but the main sources of oxygenation and decarbonation is through the overflow, sump and refugium if you have one.