During the initial fermentation some headspace is okay since you have yeast and to a lesser degree escaping co2 to scrub the oxygen out of the wine. It is when aging that you really want to eliminate any possible contact with oxygen because the wine no longer has that same protection.
What you should do is purchase a carboy, in your case 6 gallon, to rack into once your initial fermentation is complete.
Also, when topping up it is usually best to use the same grape juice that the wine is made from, or second best, some of last year's vintage of the same wine, or third best, any other of your wines that are very close in composition and quality, or fourth best, a reasonable close relation in wine, and only as a last resort would you use water to top up.
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