This is an interesting question.
For starters, high CO2 doesn't necessarily mean low oxygen, as the two are largely unrelated beyond the fact that people that use DIY CO2 like minimal surface disturbance to maximize their CO2, but unfortunately dropping oxygen levels in the process. So the two levels, CO2 and O2, are largely independent.
As far as reducing CO2, think about this analogy. Anyone who has spent much time at high altitudes will tell you that the air is thinner, and that it is hard to breath. If you haven't spent much time in the thin air and try to hike an intense trail, you will very likely find yourself out of breath as your lungs (and blood) have not yet adjusted their physiology to function at 100% in 'thin air'. Given a few expeditions though, you will find that you are winded less and less in the same environment, as your body will slowly adapt. This is analogous to your fish adapting to a CO2 rich environment ('thin' water if you will).
Now imagine some who was born and raised at high altitudes. Their body is perfectly adapted to 'thin air', and their body can get all the oxygen it needs from the atmosphere. Now, what do you think would happen if they go to lower altitudes with more O2 rich air? Not much. Their bodies are used to working in a comparatively harsh environment, but down here, oxygen is easy to breath. There is no acclimation period for them because of it. Same thing for fish. Once put in a O2 rich/CO2 poor environment (either/or), there is no acclimation period necessary because the new environment is significantly less taxing than the previous CO2 rich environment.