Older dog and puppy keep fighting, how can i stop it?

I have a one year old maltese and my partner recently bought a blue x red heeler puppy. The puppy is 10 weeks old and is already almost double the size of my maltese. They keep fighting very aggressively to the point where i have to break them up or im scared one will get very hurt. I dont really know who is starting it because most of the time they play together nicely. What can i do to stop this? The pup is female and the older dog is male (sterilized). the older dog is pretty well trained and the puppy is picking it up very fast but that doesnt seem to help.