If I remember correctly swan mussels are used, the big quoted danger
with them is the pollution associated with their decay when they die, I
also seem to remember thay have odd breeding requirements involving one
species of fish.I have an unintentionally introduced shell fish
population in at least one of my ponds but they are half the size of my
little finger nail and rather delicate, they would never contibute to
filtration.


Do not use zebra mussels or any non native shell fish, zebra mussels
are causing 'havoc' in Irish water ways and have definately
contaminated Lough Erne and have been founnd in Lough Neagh, I think
they are in the Shannon too.




--
sean mckinney