They are easy fish to keep as long as you give them enough room, they like a bit higher and harder pH than GT's but are adaptable. I would grow it out in a smaller tank, a single half-inch fish in a 125 gallon will probably be too shy to ever see, and won't eat well in my experience. The fish will eventually need to be solo in the 125, but you could try some dithers while it is growing to get it to feel more comfortable. They are piscivores so anything elongated will be eaten quickly, I might try a few convicts that are smaller than the jag (of one sex, don't want them breeding). If you have any specific questions, ask away.