Honestly I am still baffled. I've had this same CO2 rig (tank, reg, needle valve, diffuser) on smaller tanks with poor surface agitation and when the tank dumped the fish would get a little loopy but no harm done. This tank is gigantic and very slow to react to CO2 changes - it takes a few hours to come up to concentration after the CO2 turns on. So I figured it wouldn't even notice a tank dump.
Maybe the neons I had were just extra sensitive or something? Anyways I am surprised. For now I'm just going to leave CO2 off as I don't care about explosive growth and I have no pest algae to speak of. If I do decide to start using it I'll probably spring for a dual stage reg. And maybe a bigger tank, or at least a second tank.