think it will cost u ard $100+++, hv since sold the CR away. buyer of my korallin C-1501 did a simple mod on the lid for recirculating co2 as well. drilled a hole on the lid and placed an air hose adaptor to prevent co2 build up in the chamber, connecting the hose to the pump. looks good, works well so far. but again, i m not sure if it will help u with ur current tuning qns.
i m a nerd when it comes to these things.