What liquid/solution goes into a heated towel rail?

So we recently bought and had installed a heated towel rail. It came with a little heating element to heating element so that is all intsalled to, and connected to a switch in the wall.

I am wondering what liquid/fluid goes into the heated towel rail it? I have seen conflicting answers online: some blogs and stuff say you need deionized water and anti-freeze (ethylene glycol); another said normal water and something called Fernox Inhibitor is what is required.

Does anybody know what should be in there? Am I best off getting a plumber to do this or is it something I could do myself? Thanks in advance