I think garlic is used in fish foods as an appetite stimulant and some believe it can kill off internal parasites and boost immunity. I'm not sure what the science is, if any, behind this.
I don't know if garlic is harmful for newts. I suspect not. Don't forget that whilst newts are carnivorous their prey will have plant material in their digestive tract so they are quite capable of processing vegetative matter, even if they can't make the most of it's nutrition.
What is the reason for the question? Have you seen a shop selling bloodworm with garlic?
THANKS FOR YOUR INFO! Ive been feeding my newt for like 2 weeks now ... nothing is wrong with him but now he won't eat them maybe he doesnt like them now :/ Ill feed him Black bloodworms and ill try with that!
I am very suspicious of garlic supplements for non-humans. Considering all members of that particular family are toxic in some part of their structure, and that many animals suffer from the toxicity of them, I don't give my critters of any sort any product from the family.