tigathecool wrote:
my temperature is 29 degrees celcius, water perfecly clean, i change the water 100% per 2 weeks,
i dont know what to do

Ok, man... relax...no need to panic,
first thing....you just gotta
read up the articles on the website. so you have a general understanding of what you're dealing with having an oscar.
1.
You are gonna need to get a bigger tank, and fast. 55 gallon at a minimum. If you can't manage that you might need to give him up for the time being. Keeping him in a garbage can really isn't a good idea if you were to do so for any significant period of time (i.e. if it wasn't an emergency like a tank breaking or something).
I know that's not what you want to hear but he's not going to survive long in such a small tank. You might not think you can afford a bigger tank but depending on where you live there are a lot of options available in most cases...very inexpensive ones. That being said
if you are short on cash keeping an oscar is not the hobby for you. It's not cheap. At all. Even once you have all the equipment you're still paying significantly more electricity, replacement parts for the filter, chemicals, and food. And that's assuming nothing goes wrong...which it will. Oscar's live for years and years if they're healthy so you can expect to be paying a good deal of money for years and years if you truly want to get the best out of the hobby and enjoy a healthy fish.
2. do you use water conditioner when you do water changes?
3. Why do you change the water 100% every 2 weeks?
4. What kind of filtration do you have?