It fits a mason jar in the hole, and fits onto the fish tank. There are two variations of the print, one is for wide brimmed tanks and the other is for thin brimmed tanks (Not an expert on fishtanks). There are also some supports that are REQUIRED if you do not want your jar to fall into the water. They are meant to be glued on, I used hot glue. Since I am not an Expert in waterproofing, I would recommend you not use this print for over a year at a time. The plastic will break down and make your fish very sad. I used PETG because of it's good performance in water (Hence why they use PET in water bottles), but it's still not perfect. If anything about this is problematic or potentially harmful to fish, please let me know. I do not want to make the fishes sad. 

The one in the photos is a prototype since I don't have photos of the finished product.