If you have $30k in unused tuition from previous years, you put in $30k of unused tuition. There is no reason not to claim unused eligible tuition if you have it. If you put it in, and have no taxes payable, it will just carry forward to future years. If you do have taxes payable, you need to put in your unused tuition amount to reduce your taxes owed for 2017, before you can use it in the future. If you don’t include it in 2017, when you need to use in later years, you may have to come back to your 2017 return and make an adjustment. You might even have to fix several returns, depending on how many years have passed and what you had owning.