Chaeto, and all other macroalgae and plants in the presence of light.Purely hypothetical but ... What if online fish vendors used a small amount of Chaetomorpha when shipping fish?
Chaeto consumes ammonia + carbon dioxide, releases oxygen ... The only real downside I can think of is if harmful pathogens (parasites, worms, bacteria) hitchhiked their way into the bag water via Chaeto.
;Cyclops
In the dark they will use oxygen and release CO2.