Peel off the protective plastic to reveal your Lil? Alert Mate?s? yellow sensor. Place your Lil? Alert Mate? on the inside of your fish bowl or aquarium so they are peering out at you. Press the suction cup firmly to the interior of the glass. Check on your Lil? Alert Mate? daily to assure the safety of your fish. The yellow sensor will change color to tell you the condition of your water. yellow = good to go, green = time for a water changeblue = change water now!