Levamisole is supposed to be better for killing thread worms and some nematodes but is harder on the fish, it is actually a dewormer for pigs and cattle. Praziquantel is easier on the fish, especially when young, it is also supposed to be effective with a wider variety of parasites, esp. tapeworms. As Roliva said, small hairlike projections are nematodes (round worms), Pale, flat ribbons or small rectangles like bits of paper are tapeworms. Here is a picture of some tapeworms that have left some young bowfins after the bowfins were killed.

