Air space in carboy? 5 gall mead in 6 gall carboy

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Dec 4, 2007
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I am making my first mead from a kit from midwest brewers. It includes 12lbs clover honey.
It says to do a secondary fermentation in a 5 gallon carboy so that there is only an inch of space between the mead and the airlock otherwise you end up with a vinegar flavor ( from the co2?).
I only have 6 gallon carboys. Do you think this will be ok, or should I tweak the recipe to make 6 gallons of mead. If so how should I tweak it.
The kit came with prepackaged yeast nutrient and acid to make a dry mead. I do not know how much more of each ingredient would be needed for another gallon. By the way does yeast nutrient expire I have had it over a year ?

Any advice on the 5 gallon to 6 gallon disparity would be great.
I wouldn't worry about the airspace too much. Once the must starts fermenting, it will fill that airspace with CO2. Don't worry about the vinegar taste. If you want, you could up it a bit by maybe a pound or so of honey and maybe another 1/2 gallon of water. I usually have my carboys full to the shoulder, which leaves about 2 inches of head space and I haven't had any problems.
Vinegar flavor comes from one place; acetobacteria. They like to live on the infamous fruit fly. So really, what you have to watch out for most of all is fruit flys getting into your must.
headspace causing vinegar flavor??? that just doesn't make any sense. Be very sanitary (cause mead has to sit around for a long time) and it'll be just fine.
thanks for all the input.
the fruit flies sound a little freaky. I don't want to know how they would end up in the carboy.
overall not so worried about the second stage fermentation now, Thanks!
Actually headspace does create vinegar.

As you might remember from high school (though people tell me that education could be better in the USA) oxygen can oxidize ethanol (alcohol) to acetaldehyde and then acetic acid which is vinegar.

1 gallon of headspace will oxidize about 2 mL alcohol to vinegar which isn't much. I doubt you'd notice the taste frankly. I believe that acetobacter needs oxygen to make vinegar. Anyone know about that?