beeky said:
I can't really see why a sump would give off CO2, unless it was fed by a wier. If the tank were drilled and water was taken from near the bottom, fed into a sump, again near the bottom then an external filter put in there it probably won't lose that much more than a normal tank with good flow.
I think Tom Barr used drilled tanks all the time now, though I don't know if he uses a sump or not.
If you're using a sump then the water has to be taken from the top of the tank by some kind of overflow (weir, standpipe, hole in the side of the tank etc.).
If you're using a closed system (like the one I think you're referring to set up by Barr) then you can take water from wherever you want in the tank, because when you switch the pump off the water stops moving (just like how your canister filter can have it's intake near the bottom).
As a sump is fed by gravity, if you had an intake near the bottom, if the return pump ever stopped the water would continue flowing happily from the aquarium to the sump until i reached the level of the intake - which wouldn't be a pretty sight!
Having said that, I've never used an overflow in a freshwater tank myself, so I can't comment on it's effect on CO2 levels - I suspect you could make a setup with minimal CO2 removing effect. If money is no object, then using up each cylinder of CO2 quicker might not even be a problem. Just use the largest one you can to minimise the inconvenience.
I would love to have a sump based system, it's SO convenient (have had them on marine tanks before). You can put as much cheap ugly effective equipment in them as you want, can add pumps for additional turnover/different flow patterns whenever you want, and do really handy things, like put a box of floss in the sump, and just syphon from the tank into the box for as long as you like, for a quick cleanup of the tank. No messing around with anything inline (heaters, co2 reactors, just chuck 'em in the sump, and leave the pump free to do what it does best - returning water to the tank at it's max rate. Really easy to change your mechanical fltration as often as you want, and never need to disturb the biological. You can put pregnant shrimp in it to stop the fish eating the babies. I have to say, in case you haven't already guessed, I love having a sump on a tank.
And one day I'll have one again. (Not that I'm pushing for Johnb to use one, I think they really come into their own on bigger tanks - 70cm will run great on canisters.)
Mark
Mark