What "CO2 indicators" are you using?
To be honest, the HOB filters do create some surface aggitation which could degas some of the CO2, but to be honest, as long as its not a huge splashing episode, you should be ok. I know plenty of people who have HOB on planted tanks and CO2 and not many will cry over off gasing.
Don't get me wrong, there is certainly some validity to it, but in my opinion, this is one of those "internet wives tales" that has spun a nasty spell on everyone.
So, lets focus on what your indicators are and what we can do to rule out the HOB first.
If you are really concerned though about the HOB splashing the water, put something below it to keep it from splashing. Any kind of plastic can work by propping it underneath the HOB outfall.
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