This is how I will make sure I have developed a fair test

I will make sure that the containers have the same capacity as the rest and are the same shape and size. I will have the same amount of millilitres of liquid and the nails will be the same brand, length and type (iron). I will drop the nails into the containers at the same time and will keep the containers in the same room so that temperature doesn't affect the containers. I will also make sure that I will record the nails corrosion.