I did 18 liters of dark strong Ale beer
A first fermentation was during a period of a week in a bottle glass of 20 liters.
A second fermentation was inside little bottles of 355 mL each one
during 3 weeks.
To get $CO_2$ I added 6 gr per liter of sugar, but when I opened each beer I discovered a considerable amount of $CO_2$.
I do not know what happened or what was the reason of that phenomena. That recipe I used it in the past and nothing like that happened.
Could you please deduce the step when I got that pollution?