Fish fin rot and other symptoms.?

I always had this kind of problems, in fact, this is why most of my fish die - it starts with little fin rot, when i see this as soon as i can i change the water of my aquarium many times, over a period of many days. Then this does not help with the infection, it still gets worse and few of my fish gets fin rots altogether. Sometimes, usually the first few water changed, it showed substantial increase in the health of my fish, some of the rot even got away. Then always, after a few days, like 3 - 4 days, it comes back. Keep in mind im still changing the water daily. Then water changed does not help with the disease anymore.

By and by some fish will be really sick and die eventually. I have tried fungus treating medicine especially for fin rots but not once did it help anything at all, as if i wasted my money on them. Any idea what is causing the fin rot, and how can it be treated. Any suggestions muchly appreciated, as this is the only cause of my fish's death over a period of nearly five years.