The oscar is not doing well

Question
I am house sitting for my sister and brother in law and their Oscar live in a 30 gal tank by himself and he stopped eating 7 or 8 days ago.  He used to love to eat.  I tested his ammonia and pH levels and they are very high.  I have changed 30% of the water every other day for the past 6 days.  I get the levels good then they are back up again so I go through the whole process, and now he has little white worms floating in the water and crawling on the glass.  I don't want anything to happen to him and I am worried and I don't know what to do.  I don't want to dump a bunch of stuff in there.  I see in the store they have ammonia clear and pH decreaser stuff,  but I don't know if I should give that a shot or not.  I need help.

Answer
First of all, his tank is not cycled. Mention to this when they get home. Second of all, also mention that an oscar needs 55 gallons with a GIANT filter. The worms are planaria, and i bleive there are products to get rid of them. You need to continue to do massive water changes to keep the ammonia down. A good product that they make that decreases ammonia is "ammo-chips". Just pour some or all in a nylon stocking and drop it in the tank.

Let me know how it goes.
 
Thanks,
    ~Matt