Surface scum - what causes it?

[tam]

It is indeed the PAT Mini with airline option. Will adding the air mess with the co2 at all? Or shall I just open the air valve at night time?

You don't need it running full time so you could certainly just do it at night or even just experiment with a couple of hours a day. I'll let one of the CO2 buffs answer whether it would effect it running during the day.

 

[Iwagumi_Scaper]

These are a cheaper solution than the Eheim,

Perfect! Thank you AverageWhiteBloke