Yes, Koi and Goldfish can be killed by Praziquantel but the dosing has to be quite high.

Toxicity of Praziquantel by Dr Mitchell

Abstract.—The acute toxicity and highest nonlethal concentration of praziquantel (LC0) were determined
in the laboratory for grass carp Ctenopharyngodon idella and golden shiner Notemigonus crysoleucas, two
cyprinids known to harbor the Asian tapeworm Bothriocephalus acheilognathi. Praziquantel is an
anthelmintic used to treat fish with tapeworms. The 24-h and 96-h LC50 values were 55.1 and 49.7 mg/L
for golden shiners (1.3 g) and 63.4 and 60.6 mg/L for grass carp (9.1 g). The 24-h and 96-h LC0 values were
50.0 and 45.0 mg/L for golden shiners and 60.0 and 60.0 mg/L for grass carp.

The pond or tank dose of Praziquantel to kill flukes is about 5mg/L

As you can see this is miles and miles away from the lethal dose (LC50) of Praziquantel for the two fish species in the study. In fact you would have to overdose by TEN TIMES to approximate the LC50’s in the study.

Dr. Jay, for skin flukes (Gyrodactylus) and gill flukes (Dactylogyrus) in koi ponds, the most commonly used pond-treatment dose of praziquantel is:

2 mg/L (2 ppm) as a prolonged immersion treatment. (Dr Johnson)

That works out to:

The hobby often rounds this to 7–10 grams per 1,000 gallons, depending on the product and purity. (Dr Johnson)

Treatment protocol

• Skin flukes (Gyrodactylus): One treatment is often effective, but I still like to confirm with a follow-up scrape.
• Gill flukes (Dactylogyrus): Because the eggs are relatively resistant, a second treatment is typically needed after the eggs hatch, often 5–7 days later in warm water, longer in cooler water. (Reddit)

Solubility

As you know, praziquantel is not water soluble. Most koi keepers:

• Pre-mix it in a small amount of ethanol, vodka, or a commercial dispersing agent.
• Then distribute it around the pond with good circulation. (Reef2Reef)

Safety

Praziquantel has a long reputation for being one of the gentlest flukicides available for koi and generally does not harm nitrifying bacteria or koi when used at recommended doses. (Koi and Pond Services)

Greg Lewbart reference

In Lewbart’s ornamental fish notes, praziquantel is listed at 5–10 mg/L as a 3–6 hour bath, repeated in 7 days, typically in a treatment tank rather than a whole-pond treatment. (Vin)

So if you’re treating a display pond, I would think in terms of 2 mg/L (≈7.6 g per 1,000 gallons). If you’re treating individual fish or a hospital tank, the 5–10 mg/L short bath protocol is another established option. (Dr Johnson)

One thing I’m curious about, Doc: are you asking about pure praziquantel powder, PraziPro, or one of the koi-specific products like Aqua Prazi/Fluke-Solve? The exact gram calculation changes a lot if the product is only 50% active ingredient.