This morning I was taking the weeds out of my garden grass again (did this yesterday too - it never ends) and came accross an earthworm which made me jump back a foot !! I was doing some research on earthworms to see if they are friend or foe [friend by the way] when I came accross this:

Many in the aquatic hobby get alarmed when they see worms in their aquarium when in fact the majority our detritus (composting) worms that sometimes get out control due to the aquarists over feeding and poor aquarium maintenace procedures.
http://aquarium-answers.blogspot.com/2007/03/trematodes-and-nematodes-in-fish.html

Carl