You could set up a Quarantine Tank. It's a way to ensure no disease in an existing tanks. You could use a Kritter Keeper, or even a Sturdy Plastic Bin.
You just have to keep the fish in there for a few weeks. And then, if it seems healthy, you could carefully put it in the Community tank.
You just have to keep the fish in there for a few weeks. And then, if it seems healthy, you could carefully put it in the Community tank.