1) Is there anything wrong with my setup?
My aquarium is 500 liters, 5 fishes 25-35 cm: 2 wild oscars, 1 petenia splendida, 1 jaguar, 1 pleco.
Hello; You have around 125 gallons of water with a lot of large fish. The fish are messy and known to have a large bio-load. Your water change (WC) schedule and volume seem slight even if you had good source water. But with source water already having nitrates this would reduce the WC effectiveness.
Should I treat him in the common tank?
Hello; Since HITH is usually considered to be a result of poor water conditions treating the main tank with medications has the potential to make water conditions worse. I am not familiar with the medication you mentioned so could be completely wrong about this.
I usually set up a hospital tank for health issues which do not indicate a general infection.
Hello; You already are the beneficiary of advice from some of the more knowledgeable members of this forum. I do not have an answer, such as a special medication, which will allow you to keep the current procedures you use.
Two questions if I may ask. What is the feeding schedule and amounts of food?
What is the procedure used for cleaning the filters?