A kuhli loach needs a 15-gallon tank. A kuhli loach only grows to be 4 inches long, so a small 15-gallon tank will suffice for a single fish. However, you shouldn’t keep just one. Kuhli loaches feel more comfortable living in a small group of at least three. If kept alone, it might stay in hiding most of the time.
You only need to add around 5 gallons of space for each additional kuhli loach.
Additionally, this recommendation only applies to species tanks. If you wish to keep your kuhli loach with other fish, you need to consider them as well.