I don't have experience with a tank that size, but I do run an airstone on a 42 hex covered tank 24/7. I have a pH meter in there, and the last time I actually checked this, my pH went from 6.97 to 7.05 when I turned on my air (which is pretty strong aeration, Tetratec DW96-2 driving two stones). My kH is 6 dKH, so that equates to about 3 ppm CO2 loss.
It will probably be a bit more in a rectangular tank because of increased surface area. Best way to measure is with a properly calibrated pH meter and a kH test kit. I use and recommend American Marine pH meters.
If your tank is uncovered, there's really no need for an airstone if there's adequate water movement. I have NOT tested this, but I would imagine that CO2 loss will be a lot greater if you run an airstone during the day in an UNcovered tank.
I cause some controversy for saying it, but I think full stocked covered tanks (planted or not) are better off with airstones all the time. It insures high DO levels for the fish at all times and at all levels of the tank. Many people run full stocked covered tanks with no stones and they say the fish are fine. No reason to doubt them, but you really need a DO meter to know what your DO levels are. It's when they drop below 5 ppm that there's reason to worry. If you have a covered tank for example, you might think the fish are fine, but there may be times of the day where DO is dropping down between 3 and 5 ppm. This might not kill your fish, but they will be under stress and may or may not show it.