Hi Kate. They mean the same, so you can use either. By adding ‘up’, it basically just stresses it a little bit more.

‘To fill up’ has the idea that the level was down significantly and you want more. It’s the more common thing to say in daily life. ‘Fill’ sounds more technical and reinforces the idea that you want something to a specific level. Fill the swimming pool to the 4-foot mark. (To a specific level) The cost of coffee is exorbitant here. I’m going to fill my cup to try to get my money’s worth. (To a high level) Fill’er up. (Stereotypical thing to say at the gas station. I’m here because my car’s gas level is low. ‘Her’ refers to the car.) Before you continue your hike make sure to fill (up) your water bottles. (‘Up’ suggests that you think that the level might be low. Without it, you’re reinforcing that they should be filled even if the level is already pretty high.

"Fill" and "fill up" are often used interchangeably, but there are slight nuances: 1. Fill: This can mean adding something to a container until it reaches the desired level, which may or may not be full. * Example: "I can fill this pail with water." (This means you will put water in the pail, but it doesn't necessarily imply that the pail will be completely full.) 2. Fill up: This more explicitly means adding something to a container until it is completely full. * Example: "I can fill up this pail with water." (This suggests that you will add water until the pail is completely full.) In most casual contexts, both phrases can be used interchangeably without much difference in meaning. However, "fill up" emphasizes that the container will be full.

However, if it's a command, "Fill it up" is more natural. 'It' is part of the construction as well

Hi there. We generally put 'up' in a different place in the sentence. I can fill the pail up with water. 'up' isn't often used here, as it's an extra word, and we lazy English like to take the quick and easy way out. 😂