I would normally agree with reefers statement.. However, in a 12 gallon the back chamber plastic is higher then the water level and with the glass on the top of the tank the fish cannot get to the back chambers unless it is about a 1/2 inch in size and times it just right. I feel having a glass top limits uv and also gets dirty fast thus limiting the amount of light into the tank. Don't use a barrier unless you are either lazy to do a top off every other day, or you have fish that WILL jump out of the tank.
In the case of jumping fish I also agree with reef to use the screen cover. A DIY for the cover can be found online at nano-reef or it is possible someone already linked it here. Check it out
Keyword: Pond Netting, AceHardware, UV Protected
http://www.industrialnetting.com/pla...ted.htm#search
