I myself would not keep a small mouth bass in a 125g permanently.
At full size I feel like the tank would be way too small and the fish wouldn't have much room to do anything but swim back and forth a little bit.
Someone else mentioned a rock bass and I personally think that would be a much better choice for a 125g (by rock bass I mean any of the similar species). They're very interesting to watch interact with each other and their surroundings, since they will change color and pattern depending on their mood. Rock bass can also exhibit some very interesting behavior when ambushing prey, and they don't seem to be active swimmers, which is a plus for a smaller tank. Unlike some fish they don't swim back and forth all day, rather they hover in front of driftwood or other decorations. This doesn't mean they're not active in other ways, they're just not active swimmers for the most part, which means they can be kept in smaller quarters than a fish that likes to utilize the whole tank for swimming.
