I like it!
I like it.  It works exactly as shown.  The only down side is that the top portion of the rock is heavier than the simple plastic bottom so it keeps wanting to float up and flip over.  Like it's incorrectly top heavy.  You'll need to weight it down on the bottom to get it to stay facing the correct way.  Otherwise it wants to blow bubbles into the gravel instead of upward.  I used a large rock to weigh mine down.  You might want to glue on sinking weights to the plastic bottom to get it to sit like it does in the picture.
Rating: 4 stars