I have guppy fish in a 30 gallon tank. They almost never live past 2 years in captivity. In nature however, guppies live 5 years or more.
I would say that speaks more to your skill about taking care of fish then anything else. If I omit fry my fish live usually past the upper end of the age limit.
The sea world tank in San Diego is 7 million gallons and has 10 wales, that's approximately 100,000 ft^3 per whale. Further the filtration on the tank runs 30,000 gallons per minute it takes approximately 3 hours to filter 7 million gallons. Water cleanliness is not the issue, the whales and dolphins are stressed from from loud noises of children and not being able to travel, they are fed obscene amounts of antacids to try to minimize the stomach ulcers.