125 is plenty; they don't grow incredibly fast, nor do they use the space to swim. They're very solitary unless the lights are off, or it's feeding time (assuming it's comfortable enough to feed openly in the tank- which usually takes some time and proper conditions and tank mates). My point being; all you need is a decent place for them to hide; castles, PVC pipes, caves, even hamster tubes work....
BGK are very active a night and will do well in a 125g tank. Also they do better in heavily decorated tanks, with lots of places to explore. Mine is over 16", have heard of 18" but never seen one myself or a valid pic of one posted.