Cover for Snowflake eel and gas exchange

Hi, new to this site. i'm re-setting up my marine tank after moving and will be keeping a snowflake eel, so a cover for the aquarium is essential, I have a glass cover at present and if possible would like to keep but im concerned about gas exchange.
My 600 litre system has a large 3 foot sump with a skimmer in, will this be sufficient for the gas exchange