As the others have said, the first sign of CO2 issues is supping water from the surface layer.
What was the tank like that you moved them from? If it was an older and none CO2 tank, there could be differences in chemical composition of the water and pH, that plus them getting used to higher CO2 levels (even if those levels aren't enough to drive them to the surface), not to mention the environmental change (hardscape, light levels, flow changes) is likely to because them some stress. Did you drip acclimate them when you transferred them from the holding tank?
If I were you I'd stop the CO2 for the day, and turn the lights out too maybe, and give them chance to acclimatise to their new home
Hi all, thanks for getting back to me on this. I think maybe I'm more stressed with the move than the fish then. And worrying about nothing @Nick potts@Kalum@Wookii. Lethargy is defo not what they have. So that's a worry less! I was worried with topless tank they would try to leap out if they were stressed. So when they have been dancing along the glass I was worried there would be a mass exodus!! I did actually drop the water lever to give more surface agitation. And drop the co2 a smidge on the needle valve. To try to blow off some of the co2 just incase. They've been in a Biorb for 8mths. They might look pretty and they magnify the fish beautifully. But the filtration is very poor IMHO. And I chose to put an under gravel sponge layer filter in the bottom. The water has stayed clear. But the nitrates rise because of trapped detris. Its been a long 8mths of water changes and RO/DI to keep them healthy. I'm really glad to get them out of there. If anything they were much more lethargic in the orb. @sparkyweasel@jaypeecee@Andy Pierce@Libba hopefully this just means they are really very happy then, lol! They do seem a bit calmer without bubbles. But they still exhibit the same behaviour sometimes. I guess it's just settling behaviour after all. And I did wonder about reflections, but didn't think about them trying to shoal with them. So thats a possibility. Maybe they need just a few more friends. I lost 2 Danio in the orb. One when it came from LFS was behaving odd already and died shortly after arrival. And one, I think had a sparing injury which got bacterial infection that I don't manage to solve So now I'm uber paranoid I'll do something and make them suffer. So thanks everyone for advice and putting my mind at rest a bit. Hopefully new tank and better environment, they will settle in and live long and happy lives!