Main limitation is the 18" width. You don't want fish that get much bigger than 12" or so. Cichlids would be my choice, lots of cool species that could live in a 125 for life. A solo male or pair of species like Istlanum, any amphilophus, some parachromis (except jag, dovii) would be good choices. I would look at your water chemistry to help narrow down choices; hard high pH water would be good for central americans like I listed above. Softer lower pH water would be better for something like crenicichla (pike cichlids)
There are also oddball directions you could go in, like bichirs (polypterus) if you are into them