(or any new pet owner, for that matter)

When you purchase an animal from a pet store, breeder or rescue, you have NO idea what those animals have been exposed to , or sick with, prior to you bringing them home. It is for that reason that whenever you get a new animal, you should always quarrantine them as far as possible from any animals you had owned previously. This applies to fish, reptiles, birds, small mammals, cats and dogs.

I'm sorry that you lost all your fish, but hopefully you will remember in the future to keep new animals away from your current pets for at least a couple of weeks, so that you can ensure that the new additions are healthy and sound. That way, no cross-contamination will occur.