If you put 5 gallons into it, depending on your yeast, you better have a blowoff tube either ready, or already installed. If you give it enough head space, then you probably won't need the tube. Chances are, it will calm down after the initial 5-7 days of active fermentation, allowing you to replace it with a standard airlock.
When I ferment in 5-5.25 gallon fermenters, I either install the blowoff tube from the start, or have it standing by. Depending on how much wort I put into my larger primaries (6.6 and 7.75 gallon) I also have either a tube installed, or ready just in case.
Better to install it and not really have needed it, then come home to the airlock embedded in the ceiling, and fermentation spray to cleanup (mop the ceiling)... Luckily, I've not needed to mop my ceiling. At least not yet.
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