I've had my 8 or 9 inch oscar (conveniently named Oscar) for 1 year now. I got him before I knew that my 29 US gal aquarium is nowhere near big enough for him.

His picture:

I believe he now has HITH. So far treatment has been:

1. adding Aquarium Pharmacuticals (API) Aquarium salt to the tank
2. adding a Marineland Penguin Bio-Wheel 350 (just today, don't expect it to start really helping until the bacteria get estabished), left out the mechanical/chemical filters so no carbon
3. 50% water change (15 gal)
4. Adding nitraban
5. Stress Coat
6. API mineral pyramid
7. API general cure

Post maint. test results: (with API 5-in-one test strips)
General hardness: 180
Carbonate hardness: 80
pH: 7.0 (I used deionized water, no chlorine (or anything else for that matter) in it.
Nitrite: .5 ppm
Nitrate: Between 40-80 ppm (looked to be about half way between the two, guessing at about 60 ppm, was off the scale > 200 ppm before the water change/nitraban)
The test strips didn't have a specific reading for ammonia.

Aquarium setup:

29 gal. Perfecto with matching stand
Marineland Magnum 350 canister filter with micron filter, floss sleeve for mechanical
big air pump (hidden under the aquarium rated at about 1/4 HP)
Bio-Wheel 350
florescent light (removed, it died a couple of weeks ago), will have to wait until after payday to replace it)
bubble wand
undergravel filter, airstone on one side, Marineland powerhead on the other side) (yep, almost everything is from one Marineland company or another)
I don't keep any fake plants in there as they just get ripped out right away.
2 rocks with holes that the oscar used to love swimming through, but he grew...
No heater as our the water is usually 78 - 80 deg. F without it.

I'll be upgrading to at least a 55 gal aquarium, but I won't be given it for a couple of months and will then have to cycle it.

I normally feed him Hikari Ciclid Gold, but also gave him some shrimp pellets today.

Tank mate: 1 placo (Don't know the specific variety, I had him from a previous aquarium, but he is now about 7" long.

I'll probably do another big water change on either Thursday or Friday.

I was thinking of getting some Marineland White Diamond ammonia neutralizing crystals to put down the center of the micron cartridge in a media bag, but didn't know how that would interact with the Bio-Wheel once it gets going. Besides, I ran out of money to spend until after payday (Wednesday) and he already cost me about $100 today.

Forgot to include behavior...

He seems to still be happily swimming around, no gasping and eating fine.

need to keep doing water changes every two days if possible and get a amonia tester,with a tank to small the water readings will be out but i suppose u need to get a bigger tank for yr little fellow.

could u pls post amonia reading once u hv done them
thanx.

Nitrite should be zero (same for ammonia). HITH is caused by poor water conditions which you had and still have. Nitrate should be below 40 ideally below 20

You should be doing lots of water changes until ammonia and nitrite are zero. The cure you HITH is the best water conditions possible. This means lots of water changes as possible until its cured(this may take a long time) Its recommended to try to keep nitrates below 10. This is not always possible. My tap water has nitrates of roughly 10 so its very hard to keep it at 10, impossible to be below

Keep doing daily 50% water changes until your fish gets better and get that bigger tank ASAP. Also try not to feed your fish too much. Normally i would say what they will eat in 3 mins, but i would feed him either every other day or the equivalent of 2 large pellets a day

with undergravel filters it is very important to do a gravel clean regular(one side every 3 weeks or so) or they clog up and dont work