Hi Pepe,

At 120 ft by 40 ft by average of 4 ft deep
my best recommendation is to get this book:

EARTH PONDS SOURCEBOOK
The pond owner's manual and resource guide
by Tim Matson

It has just been updated (so I bought it ;-)
but haven't had a chance to read it yet.
But it deals with large ponds and their different
needs.

Mussels shouldn't hurt your pond. They are filter feeders
and they like cool, moving water which it sounds like your
pond has. We don't recommend them for garden ornamental
ponds but in your pond they should be fine.

Will they keep your pond clear?
They sure will try.
More importantly being able to keep silty runoff and fertilized
runoff out of your pond will keep it clear.
And reading Tim Matson's book ;-)

Your pond sounds wonderful!


kathy :-)
algae primer
http://hometown.aol.com/ka30p/myhomepage/garden.html