Is this skin cancer?

Question

sick fish   I have 4 betta fish and three of them I have had for over two years.  However one of them appears to be sick.  About 7 months ago I noticed a small white patch on his side.  I did a lot of reading and treated him for fungus and bacterial infections (but not all at once).  I was diligent in changing his water daily and careful not to overfeed.  But nothing I have done seems to help.  His white patch has grown much larger and now covers a large portion of his body.  Plus his top fin is completely gone.  I change his water once a week (with Nutrafin AquaPlus water conditioner)and feed him 3 pellets twice a day (and once a week I skip a day).  He has a good appetite but darts for his food so fast he splashes water all over my desk. Lately he seems to be darting around even when I just go up to look at him.  He is in a half gallon tank by himself but I don't have a heater or a filter.  He was my very first betta fish and I want to help him but I don't know what to do.  Here is a picture.

Answer
Jennifer,
The first problem is the tank. He should not be in anything smaller than 2.5 gallons. Anything smaller is not suitable. Secondly, he is a tropical fish and needs his water to be at least 82 degrees at all times. There are no heaters designed to heat a tank smaller than 2.5 gallons, and most made for that size don't work. Because his water is not heated it will be very difficult to help him. It looks like a fungal problem that has been going on for so long that I doubt his dorsal fin will grow back. I would suggest getting the right home and add a heater. You will see quite the difference when you do. When it comes to medication, it will be very hard to get the right amount in that small of a tank. I would also suggest the fungal medication made by API. It is one of the best on the market. As stated earlier, it will be very hard to treat him, if his conditions do not change. You can also add some dissolved aquarium salt to his tank. The ratio is 1 teaspoon per 1 gallon of water. This will be added to the water going into his tank when you do your water changes.