The cerges style water filter modifications work well and you can get large water filters with clear housing with 25mm PVC in/out. I would place these in the sump and use a pair of them.
They would be fed by 2 large Needle wheel powerheads, eg, maybe 2000-3000 liter hour each.
The reactors will catch the mist and larger bubbles and help them to dissolve better.
This is important because on larger tanks, the clarity is a huge issue.
They make a common clear water filter housing like this: http://www.uswatersystems.com/shop/p...-Housing-.html
Gerry, this is for you too.
You need to add a PCV tube going about 18 inches down the middle to help direct the water from inlets/outlets.
Typically there is a cartridge filter in these units, but if you use them simply as place to increase the dwell time for CO2, then you just need a PVC tube attached and glued to the middle return inside the housing.
Also, these size filters are great for water changes and to remove Chlorine etc from house tap water.
Just use high grade carbon blocks and replace every 3-4 months.
You do not need quite that much flow from the CO2 system, since the outflow from that will be fed back into the suction side of the return pump, which will do the mixing for the CO2 in the aquarium.