Hi again,
I am sorry you are having such frustrating problems. I don't believe the problems just happened; instead, it has been an issue that has been developing since the tank was set up, and it is now coming to a dramatic crux.
I would say you have to stop believing what the local fish shop tells you, as it seems they are misleading you. If they sold you all those fish while knowing the fish tank you have, they lied to you.
1- A 160 liter tank (~42 gallons) is way, way too small for an oscar, 2 parrots and 3 silver dollars. So you may need to re-consider the plan. I would say is not even large enough for any of those fish by themselves. Either, one will have to do nearly 100% water changes daily, or one will see their health decline quickly. And when (if) the fish grow, it will only get worse.
2- I am not sure, but it could be that your tank was never fully cycled. And them with such biomass and only 50% water changes weekly, things have gone down hill.
3- I suggest you choose among the fish (only one type) and re-home the rest, unless you can set up another tank. I believe a 160 liter tank is better suited for smaller fish, rather than any of the three types you have now.
I know these news are hard to take, but it is the honest true. Even if you could step up the water changes dramatically, the aquarium is still too small for those fish. In the mean time, water changes are the only effective manner of maintaining some minimal water quality.
Good luck!