Used aquarium set up

Question
I have a 30 gallon tank that was given to me. I brought it home and cleaned it inside and out.  After that, I started setting it up with the rocks, filter, etc--which I purchased brand new.
I started adding my fish, after acclimating them and they are dying left and right.  I had an albino oscar I bought yesterday, and by this morning, it was dead.  I had another oscar that was the same way.  I took them BACK to where I purchased them today and exchanged them for smaller fish.  I changed 25% of the water as well.  I had my water tested--they said it was great! No problems.
I have had several tanks in the past and have never had this problem.  I also have a 10 gallong tank that has like 5 goldfish in it(small ones) and they are all fine.  I am completely dumbfounded.
Should I drain the tank and start over completely--and buy all new rocks/gravel and go over the inside of the tank again? Thanks!

Answer
Hi Lesley,
Sounds like you the ever too common ammonia issue almost guaranteed with all new setups. I can definitely say that water quality issues like high ammonia or nitrites are responsible. Oscars can pollute their tank very quickly and cannot survive long in a newly setup tank. Perhaps by the time the petstore had tested your water the conditions had lessened. Sometimes petstores consider water with a little ammonia to be "OK"

The problem is your tank hasn't had enough time to get established. It needs several weeks for the "biofilter" or beneficial bacteria to get settled to control the ammonia and nitrites.

To help get your fish through this, water changes and using a water conditioner that neutralizes ammonia is ideal. Keep in mind the ammonia neutralizers may produce a false reading on a test kit. 30-50% water changes are the best way to go to get your fish through cycling.

What did you use to clean the the tank? Hopefully no chemicals used during cleaning the used tank.

If you haven't used any chemicals to clean the aquarium then the best thing is to just decide whether you want to fishless cycle or cycle with a few small fish (black skirts tetras, danios). Then go on from there.

I hope this helps and best of luck!
Karen~