I have (2) large oscars that are 9 years old & noticed that 1 had been keeping toward the top of the tank like he could not breathe.(gasping for air) The PH is 6.6 there is no ammonia & the water temp was 78 & I lowered it to 72-74. What could have caused his death?? I don't want the other to dye as well....HELP...

sorry you lost your Oscar. Can you please post results for
ammonia
nitrates
nitrites
these are all very important. Oscars live for around 15yrs so at a guess I would say it probably lived its life. not all will make a full 15yr. How is the other Oscar and is it showing any signs of illness? Is it at the surface too? is it breathing heavily? any signs that worry you?
I would raise the temp to 78 again if there are no signs. 72 is low for oscars. If it is breathing heavily I would guess there is a water problem or infection. If its water related we need your parameters as above. Also get the surface moving by pointing your filter at the sureface. movment on the water sureface helps oxygen enter the water.

Thank you...I have an ammonia test kit & it just shows that there is no ammonia-the test tube was clear. I have raised that water to about 74-76. As of this morning, the other fish was not showing signs of illness, but I am not home to see if that has changed. Could the temp have had something to do with the death & not enough oxygen getting into the water? I have a 55 gallon tank with a Eheim Professional II filter system.

I am not totally sure and I dont think anyone could really tell you why your oscar died. I dont think a temp of 72 for a short time would kill a oscar. Most fish illness start cos poor water. a 9yr old oscar could have just lived its life.

You say you have other fish? your tank should only contain 1 oscar cos the size isnt really large enough. What other fish do you have and how do you carry out maintenance. Nitrates and nitrites would also be a problem if present so you should also check for these. My nitrate kit is what i use most so I dont see how anyone would do without. what also puzzles me is you say your ammonia test kit was clear? they usually show shades of colour, not just clear. is that what your instruction say it would show?clear?

oscars gasp for air at the surface when the oxygen level is low. make sure you have either an air stone, or when the filter hits the water, make sure there is some sort of disturbance.