Another racking question..

I have two 6 gallon carboys fermenting, bubbles through the airlock are pretty slow now, I was thinking about racking soon.

I assume after I rack, leaving the sediment behind and probably a small missed amount of juice, how do I fill the space in the carboy to prevent oxidation?

a) add a small amount of a very similar brew/wine/juice

b) add a small amount of sugar or syrup or sweetener or honey

c) glass beads and/or marbles in the bottom

d) divide into smaller carboys

Or water.

When I make up a recipe, I usually account for topping up with water so my wine is "stronger". For example, I'll make a 6 gallon batch of must for a 5 gallon volume. Then I can add water at each racking, to eventually have it correct. But you have to plan that from the start for it to work.

I also make a 5.5 gallon batch, and put the rest of the must in a wine bottle/growler/jug/etc with a stopper and airlock to use for topping up.

If the wine is already pretty low on ABV, I'd use a similar commercial wine or homemade wine for topping up. If it was a lot of headspace, I"d just rack to a smaller carboy and put the remainder in a wine bottle and then use the wine bottle's contents for the next topping up.