I don't know if the skimmer would bring in enough ammonia to make it worth worrying about but it would be interesting to see if I could filter it if I would see a difference. Nitrates are somwhere between 0 and 5 so not too high but I only have two small fish. Maybe I'll see if I can find a way to keep ammonia out of the water change and top off water first.

Would GAC work dry? I read about a using a diy box with media that pulled air through media to remove CO2 for the skimmer. I'm wondering if something like that would work but not sure if carbon works dry.