Yes, the pucks are the 3" tablets you put in your automatic chlorinator. You can get CYA at any pool store, and usually this time of year you can get it at Lowe's/Home Depot/Ace Hardware and sometimes Wal-Mart/K-Mart. It's sometimes labled at Stabilizer or Coniditioner. Look at the ingredients though...you're looking for cyanuric acid.

It's not so much that 2-3 gallons is going to kill it all by itself. It's adding that 2-3 gallons over and over again to MAINTAIN the shock level that's going to do it. Plunking in 15 gallons one time isn't going to do it...that's allowing the chlorine level to yo-yo up and down. Think of it like antibiotics...for most things you don't just take one big dose of antibiotics and what you've got is miraculously gone...you've got to continue to take the antibiotics consistently over a period of 7-10 days to make sure the "crud" is gone.