peter, first off injecting into eheim 2213 in the long run is a no no. been there,and sooner or later your going to have issues with your filter. the 2213 is a very good filter, but too fine of a machine to inject co2 into,hard on impeller and motor. ive tried on rena xp's and they are more loosly fit and they still gurgle. your best bet is diy reactor such as barr design.with my tanks surface scum seems to be a short lived thing. when it does present its self i use a paper towel.all my tanks have small hobby pumps half way down into tank. the added circulation seems to help. just try turning off all circulation in tank and you will see scum build up fast. i would rather loose a little co2 than have the scum. regards, cornhusker