Viel Fin Oscar Seems Sick

Question
My oscar seems to be sick. I'm not sure whats going on but obviously I wouldn't be posting here otherwise.

The oscar seems to be sick or dying. He is lying on his side at the bottom of the tank. Sometimes on his belly. He moves seldomly. He rose about halfway up and sank back down lazily earlier when I got home from work.

55 Gal Tank.
0 ammonia
0 nitrite
20 ppm nitrate

I do a 25-50% water change every week as well as gravel vac. It honestly depends on how much I vac.

The fish are fed everyday, 3 times a day, in small portions.

The tank has been up and running for just over a month. However, I am positive the nitrogen cycle is complete. Between dirty water, fish and lots of gravel from another fully cycled tank.

He has another oscar tankmate as well as several red-eye and flame tetras that were used to help with the cycle. One flame tetra has gone missing in the past 3 days. I am going to pull them out of the tank now that the oscars are getting larger. Both oscars are young, between 4-5 inches including the flowing tail fin.


Answer
Hi Bill,
Could you send me a pic please? It would help me make a few predictions. Here is what i've come up with:
1. He's just lazy,
2. He hit his heat really hard ( it can make them act like that),
3. or there is some disease that you or I may not have noticed.

-Will