Death of Lucy and Ethel...my cory catfish

Question
Hi,
I recently bought a betta and 2 emerald green cory cats.  I have a 5 gallon tank, completely cycled.  I introduced all to the new tank slowly and all appeared well for 24 hrs.  Then, both Lucy and Ethel (the catfish) went belly up at about the same time.   I immediately checked all levels...ammonia, nitrate,nitrites were all absent. The pH was 7.0, temp 79 degrees. The betta is doing fine.  Do you have any idea what may have happened?
Thanks,
Becky

Answer
Hello Becky,
How long did you have the tank set up and running with water before adding any fish? Also, how much time did you allow between introducing each fish?
In order to get a properly established tank, especially such a small one, you do have to allow some time between each fish. About one fish per week is actually more than enough. Also, cycling can take up to 6 weeks to finish.
I'm assuming when you say you introduced them slowly you meant you had them acclimate in their bags for around 10 minutes before adding them. If that is incorrect in anyway, please let me know.
What kind of water were you using? And did you use tap water conditioner? Also, what kind of decor did you have in your tank?
Did you inspect the fish to the best of your ability before bringing them to home to make sure they had no illnesses (this can be hard even for an expert, as fish do hide their illness as long as possible - but some things are very easy to see - fungus, bacteria, parasites, etc.)
I will need some more details to help you figure out what went wrong. Anything you can think of that may or may not factor in, or perhaps I have already said something here that may be the cause.
Write back in if you do still need help getting it figured out.