@Kasrift
72x18x21 are dimensions. Length x width x height. The refer to the 18" as depth..front to back. Its just not enough. If your going 6 feet go 180 for the 24" front to back. If you just want the 125 gallons go for a 48x24x25. Especially if there are internal overflows on the 125 you lose basically a foot and a half in length. You can aquascape much better in at least 24" front to back...18" is tight.
I definitely feel the space lost from one overflow in our 90. It's why I'm hoping to upgrade to a 150. But needing to stay at 48" so will look at 30" front to back and 24" depth still