times 2 for each bottle, after a few hours I saw one bubble every 3 seconds, last time it went up to 1 bubble/sec where it still is now. My question is, it seems slow still and I havent had any noticeable bubbles come out of the diffuser yet.
You can use a regular air stone. You could also wedge a piece of a chopstick or wooden skewer into the end of the hose to diffuse CO2 that way, but the chopstick/skewers build up a slime fast, so you'd have to replace them/clean them off.
I'd go with the airstone in the meantime and see if you get better diffusion that way.