So my fish has been sitting under a rock/cavern decoration for about a day straight. He almost never goes in there and is always swimming near the front of the tank when I approach it. He is also sitting at the botton of the tank, with his belly touching the gravel. I fed him today and he seemed to have a lot of trouble eating. He usually sucks food up immediately, before it hits the bottom, but today he seemed to have trouble seeing it. He searched around for a long time looking for it while it was right in front of him and he ate and spit back out a lot of gravel around it before he could get the food in his mouth. Any suggestions?

First off, what are your water perimeters?

How big is the tank? And how big is your Oscar

ammonia and nitrites are good. nitrates were high but i did a 50% water change yesterday. He is a few months old and is living in a temporary 10 gallon tank until I get a 40 gallon for christmas. I know the ten gallon tank is too small but with frequent water changes (including gravel vaccums evey time) and multiple filters he had been doing fine. I also noticed that he isn't swallowing his food. Its now sitting at the bottom of the tank a few hours later.

OK, I do need more info... Nitrites and ammonia are good? That doesn't tell me anything. I need exact numbers. How are you testing?

A 10 gallon needs a daily 50% Water change. And I really hate to break it to you but the 40 you are waiting on is not big enough either. You will need at least a 55.. And that is for just 1 Oscar. No tank mates.

in some occasion, oscar become stationary to conserve energy...this means
there is a water issue.... whenever all parameters are good coupled with
good feeding habit, oscar is always happy and active.....

just a thought to share.

Can you post the actual water test results please.

How big is your oscar. The thing is your 10g is just way too small, unless your oscar is very small. As nicksue says you need to do daily water changes of 50%. Dont feed for a few days.

Do you feed you oscar a varied diet or do you vary it. If you are only feeding one food your oscar may be bored with it. If this is the case again dont feed for a few days