The only pet store close to me that keeps good care of its fish is the Pet Club. I've only seen a dead fish in their tanks once and it was 10 minutes after the store had opened so they didn't even have time to get it out. My Wal-Mart actually takes better care of their fish than my Petsmart. I hate my Petsmart, they had one tank so overcrowded it was unbelievable, I couldn't even see through the tank there were so many tiger barbs in there. There are always dead fish and the guppies I got from there had worms and I returned them.
Did you get a new filter? With a filter, 25-50% once a week. Once it's cycled, you can probably do only 25-35% a week. As you add more fish, you'll want to keep an eye on the ammonia levels to make sure the filter/cycle doesn't crash. With the four cories and one betta in my 10g, I change 35-40% of the water once a week, same with my 10g sorority of 5 girls.