All fish pee in their water and over time this builds up to a toxic level. The thing taht really hurts them is the ammonia in their pee. Filters can't always take out all the ammonia. That's why I suggest adding PRime--it turns the ammonia into something that won't hurt your fish.
Even when you have a filter, you still need to change teh water. If you have a test kit you can use the tests to check when it's time for a change. Otherwise, it's a good idea to change at least half the water once a week.
Water straight from the tap also has chemicals in it--chlorine and chloramine--that hurt fish. Prime gets these out also.
If you can't test and treat your water you can't really tell what is making your fish sick. I don't think he's turning white from old age--that doesn't happen to fish. I also don't think they get depressed. Instead, losing his color is a sign that he's stressed. The most likely stress is from bad water.