On Fri, 4 Jan 2008 20:06:24 CST, ~ jan wrote:

Anyway.... at one time I found almost all my goldfish in the pre-filter as
if the one had told the others of the big feast down the pipe. Thankfully
after a scolding, and probably growing bigger on tadpole snacks, most
stayed out of it after that.

I gather you must have frogs. I've seen my goldfish and tadpoles
competing for any bits of food that might be available and more than
once I've seen goldfish spit out a toad tadpole. I don't think they
eat them at all. I'm a bit jealous of people who have real frogs.
At first I was glad to see toads mating in my pond. I think all the
firstborn came back the following year and brought friends to the
point they were so numerous they became a bit of a nuisance and I
began to ignore them as I was cleaning and stopped trying to protect
them. It didn't reduce the numbers, there are still too many each
year, but we ignore each other and press on with living.

All comes down to design both of the filter and bottom drain style. In my
koi pond they wouldn't fair as well stuck under the screening till I
cleaned it and there is more turns in that pipe and faster flow, but the
koi are too big to get in. The lily pond set up is perfect for a ride and
fairly easy for me to net them out as soon as I discover them gone. Next
year though I expect most of the adults to be too large to fit between lid
& liner and with the anti-vortex lid, the pull isn't that strong in any
given area. Not looking forward to netting out a lot of fry.... who may
find themselves living in the prefilter for a bit growing. ;-) ~ jan

Possibly due to my short experience with koi, I never had trouble with
koi in my pipes, it was always goldfish.
The closest thing I have to a prefilter has filter screens in it at
present, and I've improved their fit to the barrel since last time I
had spawning fish so I'm hoping they will only get to the first barrel
only in the future, but I'm adjusting to whatever they do. I've
closed up the gap around the bottom drain dome and the pond liner to
about 1/4", but that is just about right for small goldfish to pass
through without slowing down and the first barrel is still open to
them. The good thing is I've made my adjustment.
--
Hal Middle Georgia, Zone 8
http://tinyurl.com/2fxzcb