Do I want to have a separate CO2 tank if I want to force carbonate? I'm thinking of trying that.
Or is that the same thing...
And do some guys have an extra sitting in there just in case they run out?
Thanks PassedPawn
Here comes a long response. The short answer is you can do it however you want.
You do not need a separate tank to force carbonate (speed up the natural carbonation process). It's just a pain in the ass, unless you make some concessions. Force carbing requires you to up the target keg to 30 psi, then ramp it down. To do this, you need some way to prevent from giving the serving kegs 30 psi.
The cheapest way to force carb on the same tank as serving kegs is to disconnect them. You can still get some pours off before the pressure drops in the serving keg, at which point you could disconnect the force carb keg and reconnect the serving keg to goose their pressure. Repeat as needed. Personally, that's the type of inconvenience I wanted to avoid by building a keezer.
Stepping up in price, if you have a manifold or distributor with shutoff valves, you can just shut off the valves instead of disconnecting kegs. This is easier, but still, not ideal. If you have a party, you'd have to man the CO2 tank.
Continuing up, the next option would be a dual-body primary. This is a primary on the CO2 tank that has two separate pressures and out ports. I went with this setup. I'm going to run the 1st primary to my force carbing keg at 30 PSI. The 2nd primary will be set to much lower serving pressures and connected to a distributor that feeds the serving kegs.
Another step up is to buy a secondary for each keg so you can dial in exactly what pressure they should get. This is probably the most expensive option, but gives the most flexibility. A 4-port secondary array will set you back $200, about the same price as the freezer.
The last option is to run two tanks, one for the serving kegs and one for the force carbing keg. If bought at the right price, a 2nd tank and 2nd primary isn't all that expensive. And it allows you to screw up and empty your CO2 tank while having a spare leftover. Note that this option takes up a bit more real estate in the keezer; you won't be able to fit 5 kegs in the GE 7.0 CF if you put a 2nd CO2 tank inside.
I bought a spare 5# tank because I live 45 minutes from the preferred tank filler, but it's on the way to work. I don't want to have to make an extra trip that far if I don't have to. Indeed, if I avoid 3 unnecessary trips, that pays for the spare tank in fuel savings. Mine won't be stored in the keezer.