In article , Sue & Bob Hobden
writes

Water Snails in a pond have the same effect as normal snails in your border,

True - land snails do a lot of clearing up for us.

they do no good whatsoever.

Not true. They do eat plants, but they also eat organic debris and
algae. They prefer soft stuff, so in practice, unless you have little
organic debris and lots of soft-leaved living plants, they won't do much
harm - a similar situation to the garden, where the amount of damage to
plants other than seedlings is minute compared to the number of slugs
and snails that may be around.

They will eat the higher order plants in
preference to anything else

No, they will not. Why eat something tough when there's plenty of soft
stuff around?

and produce lots of detritious themselves, they
certainly don't eat it!

er .. detritus covers a little more than faeces ;-)

Also a carrier of liver flukes during part of their life cycle.

Certain species of the genus Lymnaea only. AFAIK neither the nerite nor
the ramshorn are capable of acting as a host to liver flukes.

And of course, just because a species can act as a host doesn't mean it
necessarily *is* - just as I am not carrying tapeworm, roundworm, fleas
or headlice ;-)

One saving grace is that fish will eat them when small, and big fish will
crunch even large specimens. (You can actually hear it crunch from
underwater, lovely sound, and then all the bits of shell get blown out the
gills)

I don't think you and I are ever going to agree about ponds ;-)
--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/edward/index.htm