well then your float arm needs bending or you've installed it so it hits against the side of the tank .... although above you said it still showed 1/2 with the float moved manually, if now you found that the gauge properly tracked manual movement of the float you don't have any electrical problems, if it works OUT of the tank, it'll work IN the tank
the difference between a "6V" and "12V" gauge is 6V is the original electro-mechanical type and 12V is a modern electronic replacement, the difference is very obvious is you remove the case and look
Bill
