Is it lucky, or experience in water quality? In most case if the fish are healthy and crowded you can bet that the owner either has some kind of super filtration or they do regular water changes, Either way they should be testing it and monitoring it closely.
I'm pretty crowded IMO in my pond, 16 koi ranging from 17" to 6" and 8 goldfish ranging from 8"-10". in 2200 gallon swimming area but 2500 gallon water capacity, 300 gallons are not used by the fish although it is part of the system. I have digital PH, and ammonia meters and I check weekly for nitates, phosphates, and salinity. I have pretty extensive filtration, lots of water movement to keep oxyen levels high, and I do weekly to bi-weekly water changes depending on how much catch water I can obtain. My catch tank only holds about 300 gallons so sometimes I have to use city water that has been pre-filtered and run thru charcoal before it enters my pond. I usually let city water run through my holding tank for two days before using it for water changes. The rules of thumb are used to help keep fish loads at a pretty safe level with minimal water checks but I really feel that anyone that is serious about ther fishes health should monitor their water and know when something is getting out of whack before the fish tell you.
