I think it's only $1/gallon for the 10 gallon tank. And I believe that's their regular price on the 10 gal tank. If I read their sign correctly the 20+ gal tanks are more than $1/gal. Also, for some reason at Petland their 5.5 gal is more expensive than the 10 gal tank.
I'm sorry, i forgot to say that it's only for 4 sizes. 10,20H,20L, and 30