Kev wrote:
Just wondered if anybody has made a large koi pond, more than 25 feet
diameter, and fitted bottom drains. I know that they can be fitted into
the liner and the sealed with the screwed flange. Just wonder if this
is completely water tight?

Yes, with adequate clamping force, it'll be just fine.

Is the flange on its own sufficient to get a seal or do you need to use
any sealent?

Sealant is probably not a bad idea - as long as you don't use too much
of it.
There is a 'cheat', if you're worried.
You make a 30cm or so deep depresion, maybe 1m in diameter, at the
bottom of the pond.

Now put in a right-angle, and a 45 degree bend into this depression, so
you end up with the right angled bit pointing up into the middle of the
depression, and the 45 degree bend going out up the depression.
Now, neatly fill in the depression with concrete, making a nice bowl,
with the exit at the bottom.

This then either exits over the side of the liner, concealed by
something, or it goes through the liner just under the surface.