Oscar at top of aquarium

Question
I've had a red Oscar for a while now, and just got a new tank, slightly bigger, for him to move around in (55 gallon).  He's been doing fine for his first days in the tank, and eating well, though I'm still battling a fluctuating nitrite/nitrate level.  I'm also treating him for HITH.  I have a Magnum 350 canister filter with carbon media, and I was told that the filter should provide adequate aeration for the Oscar.  My problem is, on the 5th day in the tank, the Oscar is all of a sudden swimming directly at the top of the aquarium.  It looks like he's hungry, but he's not eating.  The other smaller fish in my tank (there are never signs of aggression) are at the top, too, and look as if they're gulping air.  This is just now occurring, so does this mean that I need to provide extra aeration for the tank?  I'm concerned!

Answer
Your fish are suffering from new tank syndrome, the gasping at the top of the tank is from high ammonia levels poisoning your fish, it gets into the blood stream and causes the gills to close off.  I would start with daily water changes in the amount of 20% for about a week and then go to every other day for another week, check the ammonia levels at this point and if they are still high continue the every other day water changes.  Its hard to say if they will recover, to me it sounds like its to late but I've seen stranger things happen.

Good luck.