Drinkwell Big-Dog Pet Fountain +Drinkwell Cleaning Kit + Drinkwell Replacement Filters (3 Pack) Bigger for Bigger Dogs, the drinkwell big-dog, is the only fountain with a free-falling stream. It creates an enticing waterfall that also breaks the surface tension of the water bring oxygen into it keeping it fresher. The attached reservoir expands the water holding capacity to a total of two gallons. Great for big dogs. The carbon filtration system keeps water tasting good, and returns fresh clean water over and over again.