Reservoir The Additional Capacity Reservoir and Additional Capacity Reservoir Plus, increase the amount of water the Drinkwell Pet Fountain (50 oz size) will hold. By adding the Additional Capacity Reservoir you can double the amount of water in the Drinkwell Fountain, from 50 oz to 100 oz. This is ideal for multiple pet households.   When the Additional Capacity Reservoir Plus is added you will more than double the amount of water in the fountain, bringing the capacity from 50 oz to 120 oz of water, perfect for the frequent traveler or multiple pet households.