Methanol substitute to prevent freezing?

[pyaap]

I've just had a separate baffled tank made up, in the trunk, as a water source but it has been fabricated out of aluminium, which rules out the use of methanol (even at low concentrations I think). The question I have is can anyone recommend a substitute I can use to mix with the water in the tank, for the primary purpose of preventing the water freezing (both in the lines and pump) during winter? I would like to avoid using screenwash as the antifreeze as I've read it may not be very good to inject that in large volumes into the engine. Thanks.