Sorry to hear that. Now, consider this: you didn't bother to quarantine that fish because he was the only fish in the tank...which effectively turned that tank into his quarantine. Now, he's gone...but whatever did him in is likely still in that tank, waiting for your next fish.
What to do? Sterilize the tank? You could...but then there goes the established bacterial colony in your biomedia, meaning you get to start from scratch.
Having a separate quarantine tank for new fish isn't a guaranteed way of avoiding the spread of disease, but it is infinitely better than just tossing new fish...from a place like Petco, especially...into your display tank. This holds true even if we are talking about the first fish into the tank.
What to do? Sterilize the tank? You could...but then there goes the established bacterial colony in your biomedia, meaning you get to start from scratch.
Having a separate quarantine tank for new fish isn't a guaranteed way of avoiding the spread of disease, but it is infinitely better than just tossing new fish...from a place like Petco, especially...into your display tank. This holds true even if we are talking about the first fish into the tank.