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firstly: is this NutriBAC staff useable on pacman when they lost appetite or any similar problem. Moreover if its a yes, do I dust the food or a "nutribac bath" is the right way to do it?
NutriBac is a Pro-biotic and as such, it's supposed to improve the beneficial bacteria flora of an organism digestive system. You can find more information on it's mode of use and benefits here:
NutriBAC Probiotics for Reptile Health. I think for a bath using unflavored "Pedialyte" (or similar product available in China) at a 1 part Pedialyte to 10 parts dechlorinated water ratio (80F solution) for 15 minutes will have better result.
secondly: I once read on a chat that shrimp has decent amount of nutritions, I wonder if I can feed pacman with shrimps a lot more often than other treats, and when I do so, do I remove all it's shell before feeding or just the top part on the head but leave the rest?
Based on my experience, I try not to feed any animal, foods that are not in their natural diet. Captive Pacman's perfect food are earthworms (i.e. Night Crawlers). You can alternate with gut loaded crickets or similar insects. There are also prepared foods you can alternate with.
thirdly: For monthly worm control, take Panacur for example, how to decside the proper quantity? Is it ok if I use the amount equal to the size of the frog's eyeball? cheers.
Dose is 50 mg. Panacur per kg. of frog weight.
Do not use frog eyeball diameter as a guide

. Suggest getting fecal sample done at Veterinary and then treat frog for parasites as required.