posting some good comments so far..

You should stop adding the CO2 for now and aerate the tank to see if the fish health improve. You can always get the algae off and plants to grow back but you cannot re-animate fish though. For a 10l tank, 1 bubble per 3 sec of CO2 might be tad too much but not sure if there is anything blowing off the CO2 fast in your tank or not.

Regards,
Peter Gwee