I read that to prevent parasites it is best to use red wigglers from compost (thats what I do now).
I also read that to prevent parasites it is best to keep any worms in a damp paper towel lined container for 24 hours to allow the worm to "pass" the contents of it's stomache. I guess this is due to the fact that worms eat decaying matter.
My cichlids love worms and I used to feed out of the garden but for a year or so I have been culturing wigglers quite easily in my garage.
I would be pretty cautious. Sounds like a good vector for feeding your fish parasites, fertilisers, pesticides, toxins... even if you have the worlds most pristine garden you have no idea what's seeping into your yard from the neighbors. Not something I would attempt. All this said I am not sure how exactly earthworms intended for fish food use are any better. I mean I don't know if they come from the same people who wholesale them as fish bait.
However Charles has available some live red wrigglers... I need to order some to try out on the goldfish sooner or later!
I don't know of any parasites fish can get from earthworms. I wouldn't use dew worms or worms from a bait shop; but worms collected from your garden or compost are perfectly fine to feed to your fish. The risk of any toxins being in the worms is very low unless you have been spraying your garden or lawn with weed killers or insecticides; in which case your yard or garden is toxic to you too.
Red wrigglers are the way to go. Easy to get a compost kit to create a worm factory, and you can feed them your own veggies to ensure no pesticides. I'm going to start a wriggler farm this summer with my own veggie garden.
Parasites are usually host specific (i.e. an earthworm parasit or disease would have an extremely low likelihood of transfering to a fish). I fed large South and Central American cichlids earthworms for years.
My wiggler farm has been pretty easy to care for. I had to use 2 67 l totes and drill the bottom of one of them out to allow for drainage (very small holes). There are many websites with tons of info on getting started. these woms are escape artists especially when they are not well fed so be advised.
If anyone needs a litre of worms to get started on their own colony I have lots to spare. drop me a pm if you are inerested. Free or trade but I wont be home for a week or so.
not too sure what website i was on but it said to put them in a bowl of oatmeal. just the other day i went out in the backyard and got some for my fire eels and they loved them, and they would never eat anything but bloodworms. so i am happy that they took to the earthworms
Oatmeal might work, I think that is mealworms though. for cultivating earthworms the stuff I read said to use damp "black and white" newsprint or plain cardboard. Coloured ink is toxic apparently. they dry out and cant breathe also if its dry cause they breathe through their skin.
King Worms worked very well with my Chocolate cichlids, they are apparently very nutritious. they have to be kept in bran only and not in the fridge like mealworms.