Freshwater fish illness

Question
I bought a small angel fish and added it to my 4ft tank with my other 4 angels, 2 sucker cats and 4 silver dollars. It looked healthy with no sign of disease. 2 days later it was acting strange, staying near the top right hand corner of the tank near the heater. Another 2 days past and I found it dead on the bottom. Because there was no sign of disease I assumed it was stressed even though I put many hiding places in the tank. 2 days after he died another baby angel died which I had for 3 weeks. Then today I found one of my baby silver dollars dead at the bottom near a big log ornament.
I'm really scared about my other fish. The other angels are a lot bigger so i'm hoping they wont die. I have no idea what could of killed them. This is really upsetting how an aquarium shop could of sold me an infected fish. I love my fish so much.  
I have one female angel that's a marbel who i'm really attached to. Can you please help me? I would appreciate it so much.

Answer
Hi Cathy,
Have you checked your water for ammonia, and nitrates?  Please check your water.  If you have ammonia in it, this is deadly for fish.  The only way to get rid of ammonia is 10% water changes every day until you are rid of it.
Check to see if they are getting enough oxygen.  Never fill your tank to the top...leave a space so that the filter splashes the water into the tank making movement.  Adding an air stone will also help.
For fish to die like this, it has to be one of the two things mentioned above.  Sometimes fish all die suddenly, and the reason remains a mystery.  
Seeing as there is nothing on their body, no visible signs of fungus, ick, or scars, it is either your water, oxygen, or bacteria causing a viral disease which is internal, and impossible to cure.
Hope this helps, and that you will save your other fish.
Lynda