I'd go with a community of medium sized and somewhat agressive cichlids of some type (keep them w/heavy filtration) and some fairly large dithers. Just moved mine over into a 125 from my 90. My stock is like this:
2 salvinis, breeding pair. They get mean when breeding but are usually calm and my male is actually shy otherwise. Female is very friendly and curious. She is terrifying when bolting at a dither when mad though. She's almost 7". My male is gaining mass, they get about 10". Measured him, hes 6" now. Got him at around 3.5 last year.
1 tinfoil barb, will get a few more and grow them out. I might trade them away when they get around a foot long.
3 giant danios, getting more, or those freaky glo danios
1 chocolate pleco, he does he own thing. Kinda friendly too.
1 sailfin pleco... It's p. Gibbiceps... Another recue fish. Will outgrow that 125! He's about 7.5". They can get over 2ft iirc.
1 milomo hap, very calm
1 stumpy... I think stumpy is an albino redtop zebra mbuna? I forgot the name, harder to id since he has no tail haha. Was lost since he was a fry I think. Happened with the previous owner.
I also have a convict breeding pair (currently in my 29 gallon. Gunna try to add them back in the 125 at the same time I decorate the new tank.)
Yes those are africans and new worlds, they were getting along just fine and my hardness is no problem, only problem is when the salvinis breed. No worries though since I have a divider I made from sewing mesh that I just slap in there and the dithers love eggs so they keep the parents distracted.
You could also do a very large community of livebearers. Swordtails and mollies would probably get really big with the extra space. Some can get to 6". Maybe some wrestling halfbeaks, goodeids, or even pike livebearers (if they are legal where you are) would be awesome too. Could go high tech planted and it would look pretty sweet.
You might also be able to get away with one single oscar or big flowerhorn if your filtration is up to par. Might have difficulty turning though if the tank is the standard 17.5".
