Man, I figure the average pool must be around 50,000 gallons or more. Can you imagine!!
While we are dreaming. You could do an indoor pool, say 50,000 gallons, surrounded by a greenhouse! Glasstop roof and plenty of additional light so you can control the light cycle. Huge U/V to keep things crystal clear (probably multiple is more realistic). Several huge 5000gph filtration systems. natural "soil" added onto floor and heavily plant, CO2, and add fish of choice. You could even go so far as to re-creat your own favorite vacation spot. You could create many possible biotope scenarios....and explore them with your family and friends!
Man, I need to quit....I'm drooling!