My two 125's are old Marinelands and are exactly 18.5" from front to back. Different manufacturers can use different frames though, hence needing to match the brand to fit premade tops. You'd be much safer measuring the aquarium that you have than going off of my measurements, but it will be pretty close.
And for what it's worth, mine are built into a wall. I ran a plywood support board to lie on top of the Sheetrock edge to get it flush. I badly wish I'd have put a hinged opening above the tanks to give myself front access to them. Doing all maintenance from behind the aquarium has its challenges.
Nothing good happens fast in an ecosystem.