My Oscars are not eating... please help

Question
I guess I should start by saying that I received 2 oscars for my birthday in
April.  They seem to be doing good and getting bigger.  I do have a few
questions though.  They started out in a 5 gal. tank because I was not ready
for them but a week later I went and bought a 30 gallon tank.   They seem to
like the tank and I don't think they need a bigger one yet.  I have gravel in
the
bottom and a large decorative rock for them to kind of hide.  The first
problem I saw was that they would fight over the rock, so I bought another
one, which seemed to fix that problem.  The other problem I have been
having is that they are getting bored of their food.  I started off with regular
tropical fish flakes but they would not eat them after a few days.  Then I
started a pellet I bought at walmart.  They liked that for about a week and
decided they didn't want it.  I went to a pet store and then found a food that
the first three ingredents are whole fish.  They ate that for about a month,  
but again stopped eating.  So I tryed feeder fish, only the bigger one ate them
and he would not let the other one eat any of the fish.  About a week of that
and then they will not eat.  They did not eat yesterday or today yet, I am
going to get some feeders again to see if they will eat but they will not eat
any pellets anymore.  I know they eat a lot of different things... What else
should I try?  They are about 5inches each.  
I also have noticed something white growing on the heater...  Do you have
any idea what that would be??  I test my water weekly and it is always good
but I checked it today and the nitrates is in the danger level... how do I fix
that??  What gets rid of nitrates??  Is it the carbon filter??  I am new to fish
so I
don't know what to do.  Any information for a new fish owner would be very
helpful...  Thank you so much for your time
Megan

Answer
Megan,
The nitrates must come down and the way to do that is by doing a 50% water change today, then do 25% water changes each week.
The Oscars need a minimum of 30 gallons per fish, so you need to get a bigger tank.
I don't use carbon. It causes fungus and that is what is growing on the heater. Remove it and use bio max.
The water changes will help the fungus and you could also add 1 teaspoon of aquarium salt per 5 gallons of water. Please pre dissolve it.
As for food, I always feed my oscars the sdmallest live goldfish they have.
If you cannot get a bigger tank, like a 75 gallon, you may need to keep just one oscar.