Abstract: An apparatus for use with an existing aquarium tank for extending the aquarium tank includes an extension tank having opposite side walls, a top wall, an open bottom, an interior chamber and a shoulder between the top wall and the bottom wall for supported engagement on the top of the aquarium tank. An exhaust valve assembly includes a one way air valve, a float valve and a valve seat structured and disposed to permit one way passage of air therethrough in response to an external vacuum force, causing air to be drawn from within the interior chamber of extension tank. The apparatus further includes a sealed pump box with an air pump therein to create a vacuum force for drawing air from within the interior chamber and an aeration assembly for replenishing air to the extension tank.