Choose a configuration
Oasys: capacity, stream and handling
All three current US Oasys sizes use the same basic arrangement: a plastic open bowl, a wired pump, a falling stream controlled by a physical knob, and stored bowl water that remains accessible when power stops. Start with how you will place and clean the fountain, then choose the nominal capacity. A larger number does not add a battery, automatic refill or intake records.
Three capacities answer different space and handling needs
The 64 oz PWW00-16790 holds a nominal half US gallon, approximately 1.89 liters. It suits someone who would rather carry and wash a smaller station and does not need two liters of stated capacity. If a two-liter minimum is essential, sixty-four US fluid ounces falls short; rounding it up would change the buying decision.
The 128 oz PWW00-16777 is one US gallon, approximately 3.79 liters. It clears a three-liter requirement and offers a middle option when the smaller bowl feels restrictive. It does not clear four liters. Before choosing it, consider whether you can move the filled bowl comfortably and dismantle its tower at the intended location.
The 256 oz ZWW00-16558 provides two US gallons, approximately 7.57 liters. It answers a seven-liter nominal-capacity requirement, but the larger bowl adds handling work. PetSafe's cleaning instructions suggest taking some water out with a cup before carrying a nearly full fountain to the sink or tub. That simple step is worth planning before choosing the largest size.
Retained water and continuous flow are separate benefits
PetSafe explicitly describes water remaining accessible in the open bowl during a power outage. It does not describe continued circulation during that outage. The actual continuous-flow setup uses the pump cord, inline connection and wall adaptor. If you need a station away from an outlet, compare a documented battery-powered operating plan instead of assuming an open pool makes Oasys cordless.
The top knob changes the stream and can shut it off. It must be installed correctly, and the lowest setting produces no flow from the spout. A stopped stream therefore calls for checking the knob and water level as well as the pump. The guide calls for filling half an inch below the rim and never letting water fall below one inch from the bowl base. No confirmed automatic low-water shutdown replaces that inspection.
Choose the service routine along with the capacity
The non-electrical plastic fountain parts have top-rack dishwasher permission; the pump and adaptor do not. Open and hand-clean the pump's faceplate, stator and impeller, and split the connector tube to clean its internal surfaces. Refit the pump's suction cups, cable seal and tower carefully. A large dishwasher-safe bowl still leaves these separate tasks.
Replace the carbon filter every two to four weeks and the foam filter every one to two months. Rinse filters with water, keeping soap out of their media. Check and refresh the water as needed between service sessions. Nominal reservoir size cannot promise a particular number of days between water checks.
If you want the smaller capacity but prefer a steel drinking tray and documented low-water shutdown, compare the 64 oz steel-tray Viva. It uses bubbling rather than Oasys's adjustable falling stream. That is a useful feature tradeoff to make before choosing a gallon rating: decide which drinking presentation, surface and care tasks you actually want to live with.