I have never heard of using salt for fleas - and that makes me think of the prevalence of sand fleas at the beach - which obviously do not object to salt!

Lemon is sour to us, but dogs' tastebuds are much different than ours, but I do not think that would help with fleas any, either!

I know some people use a tiny bit of garlic powder in their dog's food - but one has to be careful, as garlic can be toxic if you give too much of it to a dog!