New plec, not sure whether to treat or observe

I bought a 2" gold nugget plec the other day (LO18), and placed him in a QT tank. The tank is planted, with a sand substrate, some driftwood, and shrimp and snails for cleanup. I let algae grow on the sides before getting the plec, and he seems to be munching on it a bit, but his belly is pretty hollow and he's ignored both brine shrimp and algae wafers (added yesterday evening and the evening before). He's pooping a bit (green, which is why I suspect he's eating the algae), but not nearly what I'd expect to see from a plec. I've noticed he's hanging on the sides of the aquarium most of the time, not hiding at all, which makes me think he can't be feeling too sick.
I'm not sure whether to leave him alone and observe, or start treating with Metronidazole, any opinions?

I'm going to follow your advice, and watch him for now. He may just be really hungry - the store said they had a shipment of these fish a month ago, then another a week ago. They were all mixed in the same display tank which is heavily planted, has next to no algae and I get the impression the fish there aren't fed as much as they are in our tanks at home. Because they're all in the same tank, there's no way to know if he's been there a week or a month - if it's been a month and he's underfed, he may just be skinny from hunger and not parasites.
If he becomes lethargic or stops munching on the glass I'll try medicating him, but in the meantime I'll just keep dropping tasty tidbits in the tank and hopefully one of these evenings he'll find the food and start eating more
[edit] Holly, I'm planning to quarantine him for a month or so, longer if he needs treatment[/edit]

Biznatch said:

You should find out how long the guaruntee is at the store. When you get a gold nugget you are supposed to check if the belly is sunk in and not buy him if it is. Check planetcatfish.com to read more about it. From what I've read, they usually won't make it more than a week if they are going to die. But check the boards to read more about it.

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The store is actually the store where I work, so I don't think I'll get much of a hassle if he doesn't make it. I'm aware that it's important to check their bellies, his isn't terribly sunken (it's worse now than when I bought him, it was just flat then) and may just be the result of being in a tank with many other fish and shrimp, and being underfed, coupled with two days of barely eating after I brought him home. Before I purchased the fish I searched AC and read up about them on planet catfish, but thanks for mentioning it, it's a great site
SoCalSunset, thanks for easing my mind. He's not acting terribly ill, so based on what you guys have said I'm going to just keep an eye on him, hopefully he's just settling in.
I put a small piece of zuchinni in the tank this evening, my bristlenose goes nutso for it, thought maybe it might appeal to this plec's tastes too