Using glass marbles to cover space on 6 gallon carboy as secondary

Good morning, I am new to home brewing but very into this. I have been reading a lot and at this time I have an American Wheat on one of my 6.5 glass carboy and a Robust Porter on my second 6.5 glass carboy. I am thinking of using a 6 gallon carboy as a secondary since I am adding fruit to my final product but I have read a lot about the air space that needs to be reduced to prevent oxidation.... My question is Can I use glass marbles to reduce the space on my secondary 6 gallon carboy as it is done in wine making and with this prevent the oxidation or is there a better way?

Any help will be truly appreciated

Assuming you follow good sanitation practices, there's no reason you can't put marbles, or other inert solids in the carboy to take up space. The other options are to top up with boiled water or fresh wort(just DME/water no need to try and match IBUs). Finally Since you're adding fruit you could add a couple table spoons of sugar DME. The sugars from the fruit and any you add should be enough to create a new blanket of CO2.

Wouldn't adding water after primary water down the brew?

If your adding fruit, use the biggest carboy you can find..even 6.5 gallons is likely not to be enough depending on how much fruit you put in.

Between displacement of the liquid, and the secondary fermentation that will start, you wont have to worry about any air contact.

Have a blowoff tube handy.

no need to add sugar or DME. the fruit contain enough sugar. also, plenty of CO2 will be released from the brew due to disturbing the beer.

no probs using marbles, but be sure to sanitize. i'd suggest boiling them for 5-10 mins and then a quick star san/iodophor dunk before going in the FV.

I am so glad i asked...i have a 6 gallon that i will be using as my secondary since my two primaries are now fermenting and right about ready for secondary by this weekend.

As of today the reading on my primaries are about 4.5 gallons even though the wort was 5.5 gallons ..because of this i was a bit worried about the amount of air but on my wheat i should be fine since I'm adding a pound of frozzen blueberries ...at least i hope I'm fine with that.