So the available sizes vary depending on what can be caught. The wholesaler shipping to your pet store may not be able to ship a STF that is exactly the size you order. Lists of fish from tropical fish wholesalers are rarely accurate in the sizes that they list. This is a math question. No, really it's an arithmetic problem.
Lets do some arithmetic, and then finish with a few comments about changing water.
I like math and arithmetic. They sometimes help me take better care of my fish. You don't really have to do a lot of math or arithmetic to keep fish. But it helps, and from time to time it helps a lot. So after you remove one liter of water from the 10 gallons of old salty water there will be 9.7375 gallons of water left in the aquarium.