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When I have bought new fish I just float them and either release in tank or add water by 3rds over time and then in the tank. I've never had a fish go belly up because of the CO2 Besides if it was the CO2 you'd think it would happen in the first hour or so, as the issue is not enough O2. Perhaps because it is a diseased/weak fish the flow rate is just too much. It may have had issues before and the trip out to you didn't help. So it appears fine in the 10g as it's weak and doesn't have to handle the flow, but in the big tank it's just not strong enough. Maybe?
