Let me start with, I hope I don't offend you here. JUST going by post count, which really means nothing, I assume you are fairly new to salt? Could be way off. What do you want to keep? Large angels?...no reef. Predators?...possible softie reef but no smaller fish. Sps?...most likely fewer fish but nice sps colonies with the proper care. There is no one tank that can hold ALL your dreams together.
That said, I have a 40 lps tank, the 125 is large predator, the 150 is cold water, the newest tank is 120 and planning sps dominate. That is the next upgrade, sps and fewer fish to start. Point is that is where you get multiple tanks going, cause there is so much cool life out there to watch and learn about. Any other upgrade would be for more larger fish together.
Cold water is just that, temp never gets above 62°. Currently there are only two fish in there, though I am looking for others. One, Erosa erosa or pitted stone fish. Two, Parapterois heterura or bluefin lionfish. The bluefin hails from Japan and is a pain to find, much less keep alive. Mortality rates are staggering for this fish. Plans for this tank are a temperate reef eventually, coldwater marine aquatics will be my hook up there.
