Hi i have had this problem before,and still do on one of my tanks
What happens is i put me hand in the water and all is OK! But if my arm connects with the light reflector i get a lovely tingling sensation
so now i basically just make sure i do not touch the reflector...........
I think what happens is the light starter gets very slightly damp due to being very close to the tank so it sets up a 'live' feed to anything that touches anywhere near the lights.
Footnote this is only a minor problem on my tank so do not ignore if its more than just a tingling sensation! Remember water and electric are a very bad combination!