Any particular reason to be wasting so much medication? Metronidazole is an antibiotic btw, not a dewormer. It can treat a few parasites like giardia, though that's not something I would expect your chameleons to have. And one dose of metronidazole would not be effective enough to kill any parasites. The only thing exposing pathogens to a medication without it killing them is doing is building resistance to that medication, which makes it really hard to treat when you actually do have a problem. Deworming monthly is completely unneccessary, far too frequent and you don't even know if you are treating the parasites you actually have. If you have parasites every month that need treating then you have very poor sanitation and hygiene in your set up.
Crayola: yes you should take him to the vet and get his fecal sample testing as Alexl said. If he has parasites he will need to be treated...with the appropriate medication and dosage.