I might suggest that there is no ultimate level that will kill a fish which is adapted to that level. If I remember my numbers correctly, I have KH above 300PPM! GH is also very high but keeping in mind that the water comes from a limestone aquifer, those numbers are not so odd. Swapping female fish from tanks which are CO enriched, into tanks with no CO2 is not a problem. I used to do that frequently with mouthbrooding females who hold the fry in their mouth until they find a safe place to release them. Common practice to catch the female, move her to a quiet tank and then back to the main after she spits the fry.
The only time I saw them struggle was after moving back to the enriched tank where they tend to lay on the bottom for a while.