To change the filter on an automatic pet water fountain, first unplug the fountain. Next, disassemble the fountain to access the old filter, which often involves removing the top or housing. Remove and discard the old filter, then rinse the new filter under running water before placing it securely in its designated spot. Finally, reassemble the fountain, refill with fresh water, and plug it back in to operate.

Step 1: Power down and disassemble 

• Unplug the fountain from the electrical outlet before you begin.
• Remove the top or housing to access the filter. This might involve squeezing tabs, lifting it straight up, or detaching a specific cover.
• Pour out any remaining water from the reservoir. 

Step 2: Replace the filter 

• Remove the old filter and dispose of it.
• Rinse the new filter under cool running water for 10-15 seconds to remove any dust.
• Place the new filter in its designated slot, ensuring it is properly seated and fits securely. The orientation (e.g., black side facing the pump) may be important, so check your model's instructions if unsure. 

Step 3: Reassemble and refill

• Put the fountain back together, making sure all parts are securely in place.
• Fill the reservoir with fresh water, being careful not to overfill.
• Plug the fountain back in and turn it on to check that it is working correctly.