oxygen loss

Question
i have a tropical fish tank with healthy fish until now.need urgent advice.ive had these fish for 5 years+ and do know how to maintain them.frequant water changes,ph,nitrate,amonia levels are fine.temperature fine.but my 2 very large angel fish just died,not eating,resting on the bottom,then both under the pump system gasping for oxygen.knew they couldnt be saved by this point,loss of colour,rapid breathing and scales looking rotten.they died 2 days later.really sad and i dont want to lose my remaining fish;2 large plecos,catfish,parrot,2 huge silver dollars,red tailed shark and 2 clown loaches.did they have a disease of some type which caused the loss of oxygen or perhaps the tank is low in oxygen.the fish in the tank seem happy with no signs of the same illness but if the angel fish died of a disease,worried that the tank is now open to the others dying too.please could you give me your advise. thanks.

Answer
He sue,
Im really sorry about your fish.  Your water quality seems excellant, so that must not be the issue.  It is not the oxygen issue because you are not injecting CO2 in the tank.  Some hobbyist, like myself, inject CO2 in the tank to allow plants to grow faster.  My area of expertise is not in fish diseases, but I will try to tell you everything I know in my experience to help.  I can think of 3 things it could be.

Angelfish's have a very slim body, along with the Silver Dollar fish.  It makes them the most susceptible freshwater fish to constipation.  During constipation, their body swells and they lose their appetite.  You said they lost apetite, did you notice any swelling?  Feed them thawed smushed peas, peas in research cures a fish of constipation, we are not sure how but it does, it should be fed weekly for a source of veggies anyhow.

Fungal infections also causes the fish to be inactive and lay at the bottom of the tank.  But instead of scales looking rotten, it should have white patches.  Try to remember if there were any patches.  Maracyn works here for treatment.

Fin rot causes the fins, tails, and the mouth to rot.  The fins will be ripped, frayed, sometimes even falling off.  It will work its way to the body but doesnt always reach there before the fish dies.  Maracyn and Maracyn Plus works very well to treat this.
  
**I do want to point out something.  Ammonia toxication symptoms are gasping+impaired respirations, abnormal swimming, bloody areas on the body, increased mucus production and death.  When you said the ammonia, nitrate are fine, what are the numbers?  NitrIte (not nitrate) toxicity also show a lot of the same symptoms which causes the fish to gasp at the surface or stay near airlines or water inflow.  Test strips are very very very innaccurate and give false readings all the time compared to liquid**

Sorry I cant be of further help.
-Matt-