Apologize for the wrong data; my numbers are for a much smaller 2.5G. Still, would not keep FBTs in a 5 gallon unless for raising tadpoles/froglets.
IRT gravel comment; think the article makes it clear (and I quote) to "Incidentally, if you do use gravel, either use aquarium-safe sand, or use gravel that is too big to be swallowed accidentally."