chip wrote:

I have an almost never used concrete w/ plaster swimming pool. It was
built in 1989 and is 18,500 gal, kidney shaped 16'x32'. As a diving pool
the deepest section is about 10' and slopes from about 3'.
....
Do I fill in the bowl until I have a
uniform 3' depth?

I wouldn't. Maybe 5', but for that size of pond, I wouldn't want to
restrict it too much. I expect it has a bottom drain, so add a standpipe
to it to get it to where you want it, then add fill.

Do I have to coat the plaster to stop leeching for fish?

Define "almost never used". If it's had chlorinated water in it for all
that time, I very much doubt you need to worry at all about leaching of the
concrete and a couple of weeks of exposure should take care of the
chlorine. Even if it hasn't had chlorine in it, I can't see it being a
problem after almost 20 years.

Are there any sites that talk about converting existing pools?

You bet. Try http://www.kilk.com/pond/ or google 'pond "swimming pool"
conversion'.
--
derek