this is my first time using a secondary. i don't have enough beer to fill my carboy. i'd say i'm about half a gallon short. should i top it off with sanitary water to prevent oxidation or is it something that i should just let go?
Dont add anything, it will be fine, assuming you are using a 5 gallon carboy and it is say ~.5 gallons short. Every time you rack the beer to a new vessel you are bound to have some volume loss of some sort. This should be fine!