works good. gata change it every week tho.
another good way is to take 2 different size cups (cheap plastic) and put some vinigar in the smaller one and a hole in the bottom of the bigger one, take a string, tie a knot it one end and thread through the hole. place the larger one in the smaller one, so it wont toutch the vinigar but the string will dangle in it, after that cover the bottom of the bigger cup with baking powder, if you have a fan near it i would put suran wrap over the top with some holes in it. this is a long lasting form of co2 (over 1 week.)