I put guppys and mollys in my fuge when I'm cycleing a new tank. I set the SG around 1.019, then toss them in. They look pretty rough for a while, but generally about 80% survive and appear normal after a couple days. Then I raise the salinity up to normal reef levels, and they always seem to do fine. I've also had figure 8 and green spot brackish puffers do fine in marine tanks.
Another fun off topic story is a friends saltwater goldfish. He had just made the switch from agressive fresh to salt, and like many new to salt, he was attracted to the idea of an agressive FO. So, I talk him into at least putting all his old plastic freshwater decorations back into the tank so the fish have a place to hide and feel more natural.
He would find great fun in buying a bag with a couple dozzen feeder goldfish, tossing them in and watching the puffer and trigger devour them. One little goldfish was chased, and ran right into a castle decoration where the other fish couldn't reach him. We thought all the goldfish had been eaten, but checking back on the tank a couple of hours later, there was that goldfish, rapid gill movement, still tucked inside the castle and very much alive.
I was pretty impressed, so we netted the castle, extracted him carefully without the puffer or trigger getting him, and tossed him in the sump.
Last I saw, he was quite a bit larger, and still living in that sump. He keeps the salinity lower than a reef, but definately above the salinity of a fish's blood, which means the fish has to be either filtering the salt away somehow to get water, or adjusted to higher blood salinity maybe? (doubtful). No idea why other goldfish we put in the sump were dead in an hour and this guy lives and grows. Perhaps he is a slightly different species which just happens to look like a goldfish?
Is this common for goldfish?
