So, obviously you are going more towards an aggressive set-up if you want a lionfish. Just be aware that any fish that will fit into the lionfish's mouth (or it thinks might fit) it will attempt to eat. On that note, Volitan lions are the largest with fuzzy dwarfs being the smallest, so you can buy tankmates accordingly. What about a foxface (which is also venomous) or some type of eel? I had a large zebra moray and a huge volitan lion in my old 125 and they got along great (or should I say they could've cared less about each other) and it added a lot of variety. You might also want to consider some sort of puffer, which are really interesting. Some might grow to be too large for a 125, however. You also have to consider that puffers eat most invertebrates, so your clean-up crew will be limited. A lion will also eat shrimp, though more than likely leave snails and hermits alone. Some species of eels are considered reef safe, however, and do not bother inverts, while others might.
And then again remember to add tankmates slowly so your system has time to adjust to the new bioload. Good luck...hope this helped some!
