Mysterious fish deaths / Respitory problems

[Mikeyy]

Fish Mysteriously are dieing in my tank. This is a small 55 gallon tank which had 4-5 baby oscars, a peacock bass, and a pleco. None of these fish are over 2 inches long. They seem to be doing fine one day, eating like cows. Then one will stop eating, and hang out around the top or bottom not eating. Shortly after (a day or so) the fish will die.

My water parameters are as followed

0ppm ammonia
0ppm nitrites
15< Nitrates.

The tank IS bare bottom, and being filters by a aquaclear 110 as well as 2 power heads. I spot clean as needed.

I have lost 3 oscars and now another has seized eating and is breathing heavy....

ANY HELP???

Im thinking its something internal. But Im no expert in FW...I do much more SW....

Edit: I DID buy these fish from different LFS, but quarentined them prior to adding them to the 55 gallon...and YES this tank is cycled

 

[Lupin]

How long was your quarantine time frame?
What did you treat them during quarantine?
How do their poop look?
How do their gills look?
Do they breathe heavily?
Any other external symptoms?
What do you normally feed these fish?
How often do you change water and how much?
Temperature?