i think they are called sumps .. after some research i found some information.. but still i cant complete get the idea through my head..
Its for a planted tanks so i quess an over flow will release all the CO2 from the water through agitation ?
Sumps are much better than canisters at echanging gas with the atmosphere due to their open nature, however with a decent lid, you could probably limit it. I'm no expert when it comes to planted tanks though - my loaches uproot and shred anything I try to plant
There's a PVC overflow design that draws water from below the surface (it doesn't have the in-tank U that most PVC overflows have, it's just got one pipe that runs stright down into the tank). If I could go back and do it again, I'd drill my tank instead of making a PVC overflow.