Praziquantel treats worms - skin and gill flukes if dosed in the water column, internal worms if used in food with Seachem's "Focus".
Metronidazole treats flagellate microorganisms and some bacteria - on the skin (including Brooklynella) if dosed in the water column, internally if used with "Focus".
API's "General Cure" contains a low-ish, gentle dose of Praziquantel, and a high-ish, hard-hitting dose of Metronidazole - and can be used in the water column or with "Focus", which binds it to food.
It can be used - but shouldn't be overused - in food, with "Focus", in a reef.
It's being used in mine, as I write this.
~Bruce