Does it look damaged or unusual in anyway? I doubt it is down to lack of oxygen as all the fish would be displaying symptoms then.
It might be worth doing another big waterchange and make sure the water is chlorinated/conditioned before it goes in the tank. Stir up the gravel and suck out the muck, you will be amazed at how much crap collects there.
If there are any signs of bacterial infections or damaged fins, bulging scales etc and you can get to a local fish/pet shop, look for some myxazin and treat the whole aquarium with that.
Also what do you feed them? What type of filteration do you use? The above will probably cost more then a new fish... but I dont think thats the way to look at it.