I fear this is similar to asking how you can cure nappy rash if you can't afford to change the baby's nappy. The simple answer is you can't. You have a number of big waste producers (oscars, arowana etc) in a 10 gallon tank- this tank is suited to the smallest types of tetras, or the smallest livebearers, such as guppies and endlers. There is no way you can keep oscars and arowanas safely in a 10 gallon. They will fall ill, and eventually they will die. If I have understood your previous posts correctly, you have already lost a lot of fish to unsuitable stocking (what happened to those giant danios?). This will go on happening if you do not take the advice of experienced members. OF course, finrot is an added complication as it is more difficult to rehome sick fish. Maybe your best bet is to work really hard with daily water changes, and adding melafix and an antibacterial medication, but once the problem is under control, those fish have got to go! AN even better option would be if you could find a good Samaritan willing to hospitalise and treat your fish- and then to keep them.