Differing Fermentation rates of same beer in separate carboys?

Hey everyone,

I've read quite a bit from this forum. It has been an invaluable resource. I've finally come to a point where I need to post a question .

I brewed a 10 gallon all-grain batch (stout) on Sunday. I have 2 carboys (plastic) with airlocks setup. One carboy probably has a half gallon (maybe 3/4) more than the other. The mostly full carboy is fermenting like crazy. I had to put a blow off on it. The other carboy seemed to be getting off to a good start, but the "bubbling" has been significantly less than the other carboy. Furthermore, the krausen is significantly less than the "fuller" carboy.

Now for the questions:

Why would they be fermenting so differently? One guess would be the oxygen level... (i shook them...)

At what point should I be concerned (if at all)?

Thanks!

All things being equal, it's funny how things like this occur. Makes you wonder, huh? I wouldn't worry too much since they are both doing what they are supposed to. If anything, the "fuller" carboy had a lower volume of O2 available when you shook it so I don't think that was an issue. They are both bubbling and that's good!

Yeah, it does make you wonder... I'm curious to see if the FG comes out the same on both of these. I plan on setting them for 2 weeks in primary which will give them both plenty of time. I'll try to remember to post back my results!