"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.