Sink Twice itself is made to function with toilet tanks that are 16.8" or smaller measured with the lid off (measuring from outside of tank to outside of tank). This expansion kit allows that Sink Twice basic model (the one with several holes in it for a drain) to be used on tanks as large as 20" wide. This kit is not necessary for most of our customers who purchase Sink Twice, which is why it is sold separately. If your tank is larger than 16.8", please consider either purchasing this kit with your Sink Twice or please purchase Sink Again or Sink Bigger (depending on your tank's measurements). If you buy this through our store, we can deliver it inside of the packaging for your Sink Twice, which saves a little money over purchasing it elsewhere.