Originally Posted by venm
Oooooh, I thought so. Thank you. I was trying to keep them separate to see if I can tell if there's a taste difference between the one that was treated with a campden tablet or not. How long do I have before I should bottle?
If you can't do it tomorrow, then I'm sure it will wait a few days!
Seriously ANY headspace at any time is too much, once active fermentation has slowed. If that was my cider, I would have bottled it instead of leaving that much headspace. But it will not ruin it instantly- it's all about degrees. A degree of oxidation may give you a certain "je ne sais quio" but a greater degree may taste like sherry. An even greater degree may taste like soiled cardboard.
so, I'd top up yesterday at the latest. Then bottling can wait until you want to bottle.