One of the first fish we kept many years ago in our 75g reef was a marine betta, Calloplesiops altivelis. It was a very cool fish, but we lost it when an exotic and toxic-when-dead worm-thing died and killed all the larger fish in the tank
Anyway, we're thinking of getting another one for our 4' 120g. The only other inhabitants in the tank are a pair of ocellaris clowns (hosting in both our RBTA and GBTA

) and a solarensis wrasse. The rockwork currently in our 120g has lots of caves throughout the rock structure, which is perfect for a marine betta.
I know marine bettas can grow to 8" in length, but our first one grew very slowly over the three years we had it. I'd appreciate any comments, pro and con, about keeping a marine betta in a 4' 120g.
TIA
