Carl Beyer wrote:
OK, Take out the filtration factor and what is too much for koi?

The reason I ask is that I see so many different claims, and then I go
see a koi breeder and they have 20 big ones in 1000 gallons (30+
inches). And 50 or more in 750 gallon setups...

The main reason is, like others said, fast turnover. Those fish are not
crowded in that tank for that long. And they filter the heck out of it.
The latter point also equals risk. If they had a power failure for
an hour or two probably all those fish would be dead. They probably
have battery backup, since they know this. Do you? Lower stocking
rates means that the filtering can fail for a longer amount of time
before it's terminal.

garyr