A good rule that I have is to treat every 12months regardless. I also do regular Faecal Floats to see if there are any parasites at all. If not then no need for worming, if there is then they all get done and I try and work out where the parasites come from!
Flagyl (metronidazole) is for protozoans not worms. Be very careful using flagyl, it can cause muscle spasms. Also it can cause seizures if overdosed. I dose it at 100mg/kg and repeat in 14days. You can treat at a lower dose more regularly if needed or worried about seizures.
Panacur (Fenbedazol) is a very safe wormer to use for nematodes. It can be given orally or precloacally. I dose it at 50mg/kg repeat every 14days until clean faecal sample.
Ivomec (Ivermectin) is another, but again you have to be careful on dosing as it can be fatal at high doses. It is also toxic on Chelonians and crocs so dont give it to them!
You can give Droncit (Praziquantel) for tapeworms, I dose at 8mg/kg and repeat in 14days. It can also be used to treat Pentastomids but you shouldn't be seeing them in captive reptiles, only wild ones.
Panacur would be the safe wormer of choice from a vet clinic, you can buy others from pet shops but I dont know how well they work. Flagyl should only be used if they are known to have protozoans as you put a risk in giving it to them.
Hope this helps you out and it is just my opinion on worming
