No, kuhli loaches can’t be kept in a 5-gallon tank. Even though kuhli loaches are only 4 inches long, a 5-gallon tank is too small for them. The only time you can keep a kuhli loach in such a small tank is when you’re transporting it and you have a bigger tank ready for them to move into.
A single kuhli loach needs to be kept in a 15-gallon tank or bigger. Moreover, each additional kuhli loach requires at least 5 gallons of extra space.
You’ll need a 25- to 30-gallon tank just to keep a small group of three or four kuhli loaches.