If the air pump is positioned lower than the top of the aquarium, you absolutely need a check valve to prevent water leakage through the airline tubing. Cut the airline tubing somewhere between the sponge filter and the aquarium device, and attach the check valve in between so that the end of the check valve with the flapper (typically looks like a horizontal bar) is facing the air pump. If you install the check valve backwards, no air will flow when you turn on the air pump, so just flip the check valve around.
Best practice is to place the check valve near the top of your aquarium. This stops the water at the rim instead of down near the pump where over time the pressure could cause a leak. Make sure your airline tubing is cut nicely and free of any tears. Check to make sure the tubing hasn't dried out over time causing the connection to be loose as this will leak in the event of a power outage.