As Europeen I am not very familiar with what a gallon is in litres. I do find two different conversion factors depending on if we talk about gases or fluids.
Why do I ask?
Several on this and other forums write that they convert a 30 gallon aquarium (holds water, fluid) or use a 30 gallon terrarium (holds air, gas) and these should not be the same size in volume! Or do I miss something?
I don't know for sure, but I would assume when someone from North America is referring to 30 gallons they are using liquid measure be it an aquarium or terrarium.
A gallon is a liquid measure (3.785 liters), so applies well to aquariums, but I'd guess that as most terrariums are converted aquariums, it's just a simple carry-over.
And this might be a language thing as well, but both liquids and gases are "fluids".