Petsmart will tell you what you want to hear. It's definitely worth investing in a test kit to check your own water and get actual numbers, not just a general 'it's fine'. That tells us nothing, quite honestly.
Okay, so was the tank cycled before adding fish? Even so, you have a very heavy bioload which is giving you a lot of ammonia, especially without live plants. Most likely your fish were in the death throes of ammonia poisoning, from the information you've given us so far.
Several of the fish in your tank are not suitable for that tank size, and will either die or become very aggressive because of this. (The Striped Raphael Catfish especially). Other fish in your tank do not have suitable schooling sizes which also contributes to stress which = more ammonia and hormones being put in the water.
As a mater of practice you should always do weekly water changes of between 30%-50% and siphon/gravel vac the mulm off the bottom. For now you should look into doing daily %50 water changes, with a dechlorinator such as Prime by Seachem to renders ammonia nontoxic for 48 hours.