2.5 Gal batch in a 5 Gal Fermenter

So I want to do a smaller, experimental batch.
Bigger than 1 gal, so I figured 2.5 gal would be good. But I only have 5 and 6.5 gal carboys.

I've heard about too much headspace being undesirable, what are my options here?

Go ahead an use the 5 gal carboy? Break it up into 1 gal batches?

Would putting a layer of CO2 inside to carboy help to reduce headspace?

You can get a better bottle in a 3-gallon version that would work nicely. You can ferment your 2.5 gallons in a 5-gallon carboy though. It will produce enough CO2 to fill the carboy in a relatively short time.

5 or 6.5 gallon carboys work fine for fermenting 2.5 gallon batches. During fermentation CO2 is released and will create a protective blanket over the wort. Now you will not be able secondary in one of those because there will be too much headspace, but for primary you will be fine.

I only do 2.5 gallon brews and I ferment all of them in a 5 gallon BB before secondary in a 3 gallon BB. Have not had a problem yet.

Hmmm.. Well, I definitely want to do a secondary and I don't want to get a 3 gal BB right now.

Should I divide the secondary into (3) 1 gal batches?

I am going to get a 3gal BB soon, just not right now.

I think splitting the secondary up would be okay. Then there's the sanitized marbles approach, which I've never used.

I was thinking of doing this with an experimental batch of EdWort's. Probably 2 gallons in a 6 gallon better bottle. My theory was that since there's a fairly high gravity change from start to finish that there's also a ton of CO2 that will fill out the bottle.

I recently bottled a 3-gallon honey wheat that was in a 6 gallon primary and I moved to 3 1-gallon jugs for secondary. All three secondaries had the same FG. Just mixed it back together in the bottling bucket. It was the first time I'd ever tried that, but it seems to have worked like a champ!

I ferment small batches large carboys often. In my opinion you want to make sure the ambient temperature does not drop too much after they're done fermenting, or else you will suck back some outside air through the airlock and have plenty of oxygen in the headspace.

Also, splitting a single batch into multiple carboys for secondary-ing carries the inherent greater risk of infection, should you not be diligent about your carboys' cleaning and sanitation. If I'm bottling, I will usually not recombine them and instead treat them as separate batches, just in case one of the carboys caught an infection.