Sounds like you have something going on if fresh air makes you feel better immediately. You will be loosing some co2 from the surface of the water as well as the overflow. If you have a stable system that needs a continuous bubble of CO2 from a tank to keep the CO2 levels steady in the tank, that means that a continuous stream of CO2 is leaving the water somewhere else. I was just looking at the safety data for CO2 gas and the symptoms listed all have to deal with oxygen displacement, including nausea. It lists the chemical as not being an eye or skin irritant. On a hunch, I looked at ammonia just now, and that is listed as an eye irritant. So maybe you're getting something coming off as your tank is still cycling? Jesse _______________________________________________ GSAS-Member mailing list GSAS-Member@thekrib.com http://lists.thekrib.com/mailman/listinfo/gsas-member