I just purchased one, haven't got it yet, will let you know.
I have one of the round ones sold by one of the fish catalogs and it's garbage. The fish never go in it. I starved them for two days and I know they could smell the food inside because they would hang out at the entrance but never go in. Also the sliding door is almost neural buoyant so the door would not "drop" as stated in the directions, kind of stupid, so even if the fish did go in you had to reach in and push the door down, I'm sure scaring the fish out.