u can lose a lot of CO2 in surface skimming.
i reciently added a sponge on the bottom of my intake, and it made more water get pulled though the top skim.
This made me lose a LOT of CO2!! no im not kidding... my mix max reactor was having problems keeping it in the lime green with 2 bps on a 10 gallon tank.
if u use one.. make the surface skimming very minimal..