Allergies? LOL. The other four angels have been doing fine in the planted tank since their first introduction with the exception of that one? I'd think it were the single fish itself rather than anything with the tank if the other ones are fine.
I think this must be it, just b/c water has good oxygen, the fish still is not used to the co2 levels(they coexist at independent rates).Maybe you can turn off your CO2 and do a WC before moving it to the main tank,then let it acclimatize to the CO2.
But did you do it durring the day when the CO2 was still on? So it would build back up pretty fast.That is exactly what I did. Changed about 70% of water, then moved the fish. So the CO2 was low when transferred.