Well, looks like one Purple Tang made it through QT with some intestinal parasites still living. It was skinny on arrival but otherwise enthusiastic about everything. Gave it two doses of GC in QT, and then CP, but now in the display it's still very thin despite a ravenous appetite, and yes it is displaying intermittent WSP. (Doncha love all these acronyms?) Which isn't a huge surprise as I didn't use medicated food, just the metro in the water.
Aside: I've also noticed defects in its dorsal and anal fins which could be nips although it seems I've read somewhere that nutritional deficiencies (like you get with parasites) can also cause this, although I can't find the source now. Anyone else know of something like this?
Anyway, so now I'm going to use either metro or GC, mixed with Focus and frozen food, to try and eliminate this. Should be pretty easy, with time, as this fish is very hungry and loves meaty foods as well as algae. I was going to follow
@Humblefish's recommendation (thank you sir!), but before doing that, I wanted to consider GC a bit more.
First, what are the actual concentrations of metro in the two products? -
Metroplex is 70% metro, or 700 mg metro per gram of powder. 1 scoop of Metroplex is 0.22 grams of powder, or 154 mg metronidazole.
GC has 250 mg metro per packet. Each packet is 2.1 g of powder. Concentration of metro in GC is therefore 119 mg per gram powder. Much lower than with Metroplex. To get the same amount of medication in your food mix, you would need to use 5.9 times as much GC as Metroplex. But how much is actually needed to be effective? Could you use a similar amount of GC, resulting in much less metro in the food, but have it still work and also be less hazardous for inverts? I don't know, but I don't think I'll try it this time, I'll just go with Metroplex as recommended.
Another question about GC when used this way: Praziquantel is pretty much safe for fish externally. Is it known to be safe when ingested with food?
API is a lot more transparent about the metro found in GC: 25mg/gal. That's right at the high end of the recommended dosage. Now, whether or not the metronidazole found in any of these products is pharmaceutical grade is highly dubious. 
I buy generic meds from Diamondback Drugs, Fishman Chemical, National Fish Pharmaceutical and Fish Remedies LLC, and they all stress to me that their products are pharmaceutical grade and purity is close to 100%. But unless I am willing to pay for an expensive chemical analysis, how do I really know?? 
