Just transferred my Tangy's from my 46g into their 125g and wondering what to put into the 46g. I kept a 6" featherfin synd. in it to keep the bio going. My first thought is to use it as a grow tank for my 125g because at the moment it looks a bit empty...fish wise. I do know that the Dolphin and Front are going to get larger but I feel I have some room to work with considering I only have 6 fish in it.
If you are planning to continue adding to the population of the 125 gal. Eventually situations will arise where the smaller tank will come in handy. I use my smaller aquarium as a time out tank for misbehaving fish; growouts & occassionally as a QT tank.