In photography what is used as fixing agent?

The agent is a "thiOSulfate." There are two in use.

The one we all started out on, because the other is more expensive, is sodium thiOSulfate. It comes as a powder you add to water, stir until it dissolves then add more water to make the recommended amount. There's normally some potassium alum in it to harden the Gelatin (fixers for color work don't need this as the emulsion is prehardened in manufacture) and some acetic acid to act as a stop bath.

The one we all should be using is ammonium thiOSulfate. It's a liquid so it's easier to mix, it works faster and it's easier to wash out of the emulsion. It also contains hardeners and acids.