Oscar tank ... NEED HELP !

Question
Hello there, I have started a new oscar tank and its a 55
gallon. I have 4 oscars (2 tiger and 2 albino) and 2 picas
catfish. The tank was ran for a week using 2 aqua clear
300's. The filters were setup with sponge on the bottom and
carbon on top. I then added the fish in the 2nd week. The
first few days were fine then things went horrible fast.
I tried so many things after I saw one tiger Oscar acting
weird. He was not eating or swimming with the rest. I was
told by my local fish guy to test the water with the 3
tests. I tested the ph first which was 6.8, I then tested
the ammonia which was between .25-.50 , then i tested the
nitrite which was 0 . I know that the ammonia was high so I
did a 50% water change and tested again, the same thing. I
added Prime water conditioner to the water and tested again
... same thing. Well about this time i was just fed up. I
then added some aquarium salt and ammonia tablets to bring
down the ammonia. I tested the water after and nothing, it
had the same level as before. I don't know what is
happening to my tank, its driving me crazy. I lost 2 fish
today, one oscar and one catfish. They seemed to have a ick
like thing on there fins. I removed them and treated the
tank with ick cure. I don't have a clue to what is killing
the fish, I have tried soo many things. I have tried to
reach out to so many people but what they say doesn't seem
to work. The way the oscars were acting was, they would
rest on the bottom of the tank and breathe very hardly.
I hope you can help as I am about to just give up this
hobby all together.

PLEASE HELP,
Shawn

Answer
Hello,
Your tank must run for at least 3 weeks before you put a fish in it.  You must test your water and when all is perfect, then you add one fish...you wait another week, if it is still perfect, you add another fish...and so on.  When starting an aquarium, you must let it run...a good thing to do is buy a used sponge from your Pet Store, and put it into your filter when starting a new aquarium.  A used sponge is full of good bacteria, and helps with the chemistry of your water.  No fish will survive in a tank that does not have the right chemistry or bacteria for them.  This is a common mistake that people make when starting a new aquarium.  You must be patient.
One Oscar needs at least a 66 gallon tank, 2 oscars 120, 3 oscars 180, 4 oscars 250 gallon tank.  Putting 4 oscars in a 50 gallon tank is not right.  They grow to be big, and need space to swim, and stay healthy.  They do not take time to grow, and once they are big, no Pet Shop will take them back.  The Oscar is one fish that is really mistreated...to know fault of your own, but to the seller that knows better than to sell Oscars to people who don't have the right size tank to care for them.  They end up sick, their growth is stunted, so their internal organs cannot grow, so they die.  It is a money making machine, and it just boils me over.
You are also lacking oxygen in your tank.  When fish stay at the bottom or top breathing heavily it means that there is not enough oxygen in your tank.  Never fill your aquarium to the top, stop adding water at the safe line, so that the filters will splash into the water causing movement.  Sometimes an air stone will help.  Ammonia is the number one killer of fish, it should always be down to zero.  No product will help, the only way to get rid of ammonia is water changes every single day of 15 - 20%.  
This is what is wrong, too many fish added at once in a newly setup aquarium where they didn't have the right bacteria, and the chemistry of the water wasn't right.  
Please think it over before keeping Oscars, there are many other nice fish that would be happy in a 50 gallon tank.
Hope this helps
Lynda