Some fish can adapt to about anything and do fine. I have a clown loach who has been living alone with guppies in a 75-gallon tank with a little freshwater salt in the water and a crushed coral substrate for going on 7 years. He's happy and doing fine.
Oh teh noes!!! He needs slightly acidic, soft water and a soft substrate for his barbels plus other clowns to socialize with! Bah. He started out as one of three, but the other two died after two days in the QT tank (actually had soft water, slightly acidic, and a soft substrate of play sand), but PetSmart loaches seemed to all have parasites and die before I could cure them. I finally got his infection cured with anti-parasite food and got him slowly adjusted to the new pH. A pH of 7.8, rough surface, and water hard as nails haven't hurt him in all these years, and he's very happy. He cleaned out all my snails (good for him), regularly hangs out inside or on top of a giant fake stump in the middle of the tank, and clicks so happily when I feed him tubifex worm blocks that it sounds like the tank glass is breaking.
My knight gobies? I was actually relieved when they all eventually died of old age. They were the meanest fish I've ever had other than African cichlids. They fought amongst themselves, they ate mollies (one nearly choked to death on a half-grown silver molly fry, with the tail-end hanging out for a long time), and generally terrorized everything with fins in my brackish 75-gallon tank. If I didn't have places for them to hide, I think they would have fought it out survivor-series until death all at once. I don't like mean fish, and trying to give them diversionary cover like large ornaments, tons of fake plants, caves, and rock walls didn't do anything. Still mean. So, I can vouch personally that knight gobies love small fish. Even when you are chasing them with a net, trying to catch them to sock them in another brackish tank to save your precious mollies, they still have the nerve to eat the fry right in front of you. I kept my 75-gallon tank at a specific gravity of about 1.010 (roughly half saltwater), and they loved it. The mollies loved it too, all except for being nipped at constantly and having their babies eaten.