While it is true a canister is more efficient at Co2 usage and a tank from what I hear can last for years, maintaining Co2 @ correct levels is possible without much Bps in a sump environment if proper diffusion method is used
, I know this because I am doing it. I am maintaining a limegreen DC with only 5Bps and my setup consists of a 90gal, a CPR CS150 overflow in which I have placed a Durso standpipe, not to retain Co2 but too reduce noise and a 30gal sump. The water from my tank is not directed underwater, it is filtered through a micron sock which gets dispersed over another layer of coarse filter media, then finally over a Biowheel and I have no special covers to retain Co2.
IMO I think the sump / Co2 loss explained above is exaggerated and you do not lose much Co2 as stated. If I can maintain a limegreeen DC with 5Bps on a 90gal how much less is needed in a canister setup?
While I will suggest a canister filter for ease of use over a sump don't be discouraged in using your sump on your tank as needed Co2 levels can be maintained. Take a look at this Reactor
, this is what I am using currently with excellent results and prior to this I was misting with a Rhinox 5000.
Now the important thing with a sump setup is the final flowrate that is used 'tank turnover rate' this should be around 600gph with less being better
, anything over and you are asking for trouble, remember this is not 'saltwater' :-D.
Here are some plants in my 90gal sump setup....see my sig also........
When I was misting with Rhinox as you can see healthy unstunted growth
Using a sump on a planted tank is possible as I am using one now and I have no fancy setup to prevent loss of Co2.