Welcome to maast. plenty of us are wannabes compared to some of the experts on this forum. I'm sure you'll get everything figured out real fast now. Sometimes stores prey on beginners which I hate to say, but they never should have sold you some of those fish. It's hard for most people to keep mandarin gobies because they require an established tank with lots of live rock. When fish are dying the solution isn't replace them with another, but saying no means not making a sale which takes precedence over giving proper advice in some stores. I think almost everybody has had this experience starting out. Find a good store that has people who care about doing the right thing. check out the vendor experiences forum to get an idea of where to go from here. good luck with everything.