Scale distension can be a sign of external parasites, or one of the digeneans life stages. If the healthy scales lay flush, un unhealthy scale looks akin to a ruffled feather. I wouldn't use prazi in a tank with inverts as Ive never tried and wouldn't advise it; molting crustaceans might have sensitivity to the meds. Corals would rerspond defensively, causing stress but I don't think theres toxicity. Seahorses should be fine, but I would check with a seahorse authority, bc I am not. For external parasites buffered FW dips are effective. But only against the larva attached to the fishes epithelium. This addresses only one stage in its life cycle. For example when treating digenean flukes in my previous location, I would first acclimate the fish to hypo salinity ( 1.009 ~1.012). Then I would administer Copper sulfate; I liked copper power because it had long term water solubilty unlike cupramine. The for the two weeks they were in treatment I would administer praziquantel solubilized in cheap vodka and administer a bath. The length of the bath was different depending the type and severity of infection as well as overall health of the fish during treatment. Hypo eliminates a majority of the larva, copper sulfate eliminates the remainder of the larva and young adults, while the prazi once in the fishes system will cause adults to drop off. Point being there are several treatment methods available, just be cognizant of which life stage you are targeting with your treatment and which may still pose a threat.