The picture isn't really clear enough to say with certainty - but I think
@vetteguy53081 has it identified correctly - Pisicolid leeches. Praziquantel won't help with these. Here is an excerpt from my upcoming fish disease book:
Leeches (Pisicola, Myzobdella, Piscicolidae and many related species)
Cause
Leeches are segmented parasitic worms that belong to the phylum Annelida and belong to the subclass Hirudinea. Common fish parasites seen in wild collected freshwater fish, they are rarely seen in marine aquarium fishes. They are external blood feeders and can be found on the fish’s fins, skin, gills or mouth. They have a sucker at each end, and are segmented. This differentiates them from trematodes.
Symptoms
In virtually all instances, aside from the visual presence of the worms themselves, the only visual symptom seen would be general anemia. Leeches are also suspected of transferring other diseases to fish due to their feeding habits.
Diagnosis
Except for cryptic gill-dwelling species, leeches are easily diagnosed by visual exam. In many cases, the leech will be attached by only the feeding sucker, with the other end of the worm free. Goldfish and koi held in outdoor ponds frequently develop leech infestations in the spring. In marine aquariums, various gobies seem prone to leech infestations.
Treatment
Manual removal is one treatment that is often suggested for leeches. However, due to the physical damage that can occur while restraining a fish and removing these parasites, chemical bath treatments are preferred. Remember however, as with trematodes, if the leech infestation is severe, and the treatment causes all of the leeches to leave the fish at the same time, many small lesions will be present in the fish’s skin. In some cases, blood loss from these open lesions, combined with the fish already likely being in an anemic state, will cause mortality.
A common treatment suggested to control leeches is the organophosphate insecticide, trichlorfon (Masoten, Dylox). The typical dose given ranges from 0.75 to 1 mg/l. This chemical is very dangerous to humans and can cause adverse reactions in sensitive fishes. Therefore, its use is not recommended here. A slightly better alternative (not as effective and only slightly less toxic) is a high dose formalin bath. In warmwater fishes, the treatment would be 150 to 167 ppm formalin in water for 45 minutes to one hour, under good aeration. If fish are suspected to be sensitive to formalin, elect to use the lower dose and the shorter time frame to begin with. Subsequent treatments will be necessary as many leeches are egg layers, and second or even third treatment will be required after the eggs hatch out. For sensitive fishes, an osmotic differential bath can be attempted (5 to 7 minute dip in either freshwater or seawater).
Jay