New Tank Problems

Question
I recently bought a new 20 gallon freshwater fish tank. I had the tank set up on Thursday, August 30th and waited 3 days until I added some fish. I had the filter, heater, tap condtioner, everything was ready. So, when I went to my local fish store, I purchased:
-a red tail black shark
-3 silver dollars
-1 ballon molly and her 3 babies

Now, I have not yet tested the PH, nitrite, ammonia, nitrate, ect. The night I purchased the fish, my black shark died and 1 silver dollar died. I was puzzled. I have done a water change tonight. But my fish are not eating. Can you please help me? :(

Thanks :)

Answer
Hi Jessica, my suggestion would be bring a sample of your water to your local pet store. They will usually test it for you for free. And if something is wrong with your water they will instantly supply you with the right things you need to fix it. I would go as soon as possible to get that done though because it sounds like one of your levels may be up and it can effect your whole tank. The reason your molly and her babies didn't die was because they are very hardy fish. Good luck Jessica let me know if you have anymore questions.