How to make sure live food culture doesn't carry any "nasties"?

[awong918]

I ordered some scuds to culture as live food. Unfortunately, they also came with planaria, hydra, (possibly) leeches, and likely other various organisms.

I'm not too worried about the planaria and hydra (I don't keep shrimp), but how do I make sure there aren't actual harmful things like fish leeches, tapeworms, or harmful parasites?

[Guppysnail]

Quarantine. I put them in a clear container on my windowsill with some plants and a small piece of wood or leaves from a tank and an airstone. I use my kitchen sink window so I can carefully inspect daily for critters. I grow my scud colony 30 days beyond the last sighting of a nasty. Most things will have gone through all life cycle stages at 30 days for me to see and remove. 
If they come in heavy with things as yours appears to have I do a bit more. 
I scoop them individually into a small dish of water for a rinse then into the grow out to try and limit the nasties going into my grow out container. 
 

Being summer I do have a piece of cardboard to place between the container and window to avoid  the direct sun cooking my scuds. 

[Guppysnail]

Here is a journal I did on live foods. My scud quarantine is on the second page. Here is a photo the long one is the scuds and the juice jar are my seed shrimp.