[inspired by swimr's idea]
The pumps in fish tanks act to oxygenate the waters to ensure that your fish don't expire, but if you're one that doesn't constantly watch your fish tank, it's difficult to know whether the pump has been working all day to keep the waters fresh.
The pump error warning
clock will help you.
As the pump works, little air bubbles rise through the water - the clock is placed just above the pump with it's face pointing outwards displaying the time.
As the bubble rises, it hits the paddle of a small 'watermill wheel' within the clock mechanism. This turns the wheel and drives the clock.
The fish owner sets the clock to the right time and places it back onto the pump in the tank. If the pump has worked properly all day, the clock will show the right time on your return.
It's says 3:45pm! Poor little fish!