Water should be room temperature for this DIY. If water is too cold or too hot, the polish won't float. Though it's important to note, some of nail polish may sink or disperse slightly on your first few pours. It's helpful to pour slowly and just add a little bit of polish at a time. Once you start to create an initial film of polish on top of the water, it seems to create a thicker surface that helps "support" the rest of the polish you'll be adding.