On behalf of the mods, long time members and amateur microbiologists everywhere, please read this post with some levity, while I attempt to be firm, but gentle.

In regard to the sheer volume of pictures posted on this subreddit concerning the growth of some mysterious blob on the top of one inch of liquid in a one gallon carboy, there is only one answer you truly need to be aware of…

HEAD SPACE = DEAD SPACE

If you are going to attempt to ferment any beverage, of any type, for any reason, you must, must, must use enough liquid to completely fill your fermentation vessel. If you do not have enough liquid to fill a container, get a smaller container. If you cannot get a smaller container, you must get more liquid. This is not optional.

Why is it important? Microorganisms.

What are these microorganisms? Anything that is not Saccharomyces cerevisiae ie the brewers yeast you added.

Where do these microorganisms live? In the empty space in your fermentation vessel, also in the air and literally everywhere on earth that has air.

If I fill up my carboy all the way full, won’t it overflow when fermentation starts? Probably, yes. Simply sterilize your airlock after overflow and put it back on. This is far superior than headspace.

Once you have successfully completed a few batches, you will learn how much headspace is acceptable to compensate for high kräusen, CO2 blow off and good sterilization practices that can help you find tune this process.

Part of the fun of fermenting cider is how incredibly easy it can be. It’s good to try things and learn through experience. However, please heed this simple PSA: fill your vessels up all the way. All that extra space you’re leaving is teeming with airborne micros and is very easily avoided.