Within 4 days of adding those petco fish, almost all of the fish in my 30g are dead - only a small 1/2 beak and 1-2 kuhli loaches (hard to keep track of) are still alive and seem unaffected. I quarantined the sick fish once it started, and treated for ich and added stress coat, but it was too late. I was able to identify ich, fungus, and Neon Tetra Disease, the latter taking out the tetras and the guppies rather gruesomely. However, it's possible other diseases were present, as I still can't identify what killed the upside-down catfish.
So ... I guess it's time to start over.
But I'm a little paranoid now. Is the tank still able to support life? Is it just a death trap full of pathogens? I did a 90% water change when I quarantined the sick fish, but had more fish die in the 30g anyway. How long should I wait/ what can I do to ensure the tank isn't a death trap?