The best way to have a sump with minimal CO2 loss, is to have the sump and the tank on the same level. Instead of having the sump underneath the tank you place it beside it or behind it. This, of course, takes up more horizontal space but does have a few advantages.
The first is that it is impossible for the sump to overflow in the event of a power outage. Even if you just had a direct siphon from one tank to the other, they would just balance out.
The second is the lack of head. Since you don't have to push water up 4 or 5 feet, fighting gravity all the way, your pump if far more efficient. Because of this you have the option of either getting a less expensive pump with lower head, having an enormous amount of throughput from a big pump, or creating head for the big pump by using manifolds and multiple small outlets placed throughout the tank.
Plant tanks enjoy the benefit of not needing large sumps, and oxygen exchange towers. We just need a place for unsightly equipment, and some mechanical filtration. With a tank that is 30" FTB you could easily have a sump that is 6" x 30" x 18" and have more than enough filtration. I recommend machine washable filter socks.