My theory of fishkeeping is to recreate "natural" conditions as closely as possible. Fish immune systems aren't designed to cope with chlorine from tap water or soaking in high concentrations of fish waste.
I feel if I keep the water conditions close to nature, the fishes immune system seems to be adequate. At least for the most part.
Temperature variations, current changes, addition and removal of other fish, irregular feeding are all part of living in the wild. Fish that have been raised at exactly 78 degrees since hatching, will not cope well with a three degree drop in water temperature.
That's why I don't feel guilty leaving the fish for a weekend without feeding, or making adjustments to the landscape. It's all part of the fish life experience
My sister-in-law (from whom I inhereted my current tank) kept rainbows for 6 years without ever doing a water change. She dechlorinated her water, and topped it up when it evaporated enough. She added this chemical called an Ammonia Bonding Agent, which basically is supposed to remove ammonia by bonding it into harmless compounds. This is frequently used for transporting fish in small containers and such, but I've never heard of long term use of it instead of H2O changes .... I shudder to think what the concentration of those "harmless" compounds must have been after 6 years

But the fish were fine... maybe they developed a resistance to them
