I'm sorry for your loss I can't imagine losing one of my frogs.

I don't suggest buying animals from PetSmart. From what I've heard, breeders are the best way to go.

I'm thinking that your frog and goldfish died from ammonia/nitrate poisoning. If you are planning to get another frog, I would clean out the tank thoroughly, place everything back in, including the filter. Fill the tank up with conditioned water (de-chlorinated/chloramined) and cycle the tank. Here is a page on the cycling of tanks:

How to Cycle a Fish Tank - Oscarfishlover.com

As for the maintenance of the tank once it is cycled, I'd do 25% water changes 2-3 times weekly. I'm no expert on these types of frogs, but I know that even small amounts of nitrate and ammonia can and will kill them.