I have no experience treating seahorses with toltrazuril specifically, but I suspect that they will tolerate the general fish dose very well.
It has been my experience that Hippocampus need not be considered at all delicate when it comes to disease treatments. Seahorses usually tolerate all the usual chemothberapeutics well, including copper sulfate, formalin, malachite green (or combinations thereof), and methylene blue. They typically handle antiparasitic agents (e.g., metronidazole, fenbendazole, praziquantel, and dylox) with no problems and do well at both hyposalinity and hypersalinity when it comes to osmotic shock therapy (OST). They can be treated readily with most antibiotics regardless of whether they are injected, ingested or administered as baths.
In short, I have found that Hippocampus generally tolerates the same treatments and standard dosages used for other finfish and marine teleost fishes.
The only medications I specifically avoid when treating seahorses are copper sulfate and chloroquine. Although they tolerate copper sulfate well, it has the unfortunate side effect of suppressing the immune system as well as the usual difficulty of maintaining the proper therapeutic dose, and there are now safer medications and treatment options that are more effective than copper sulfate for treating Cryptocaryon and other parasites. Some syngnathids have been reported to be sensitive to chloroquine, so I avoid that as well.
In short, I would not hesitate to treat seahorses with toltrazuril at the appropriate dose for other teleost fishes of similar size and weight.
There is a reference on the subject of myxozoal problems in fishes (Histopathology of Protistan and Myxozoan Infections in Fishes, by Iva Dykova and Jiri Lom) that you may find helpful if you don’t already have a copy, Bruce. See the following URL for further details if you are interested:
Best of luck treating the group of seahorses with the myxozoan infection, sir!please keep us informed regarding the results of the treatments