You'd have to measure the inside of the breeder first because they aren't always the same size. The best thing to do (what I plan to do always) is take the viv to a tile specialist and have them measure and cut out the exact size. It would be so much easier, albeit expensive, but the tile from Home Depot or Lowe's won't fit correctly because their tiles are typically 12x12 and so you would have to cut it anyways and they don't cut them any more. Even the 18x18 are slightly too large. The measurement of the tanks are usually 18x36 on the outside and not the inside... At least that is what I have found.