"Deep" or often referred to in measurements as the "Width"
Length x Width X Height
AKA
72 1/2 x 24 1/2 x 25 5/8
"24 1/2" would be what he's referring to, and in this case, it isn't an "Extra Deep" tank, it's a standard 180. if it were a 30+ inch wide tank, it'd be 'deep' in his thoughts, I believe.
Aka, Front to Back