How do I maintain healthy boundaries if my parents are fighting with each other?

In every relationship there are fights and arguments it's normal. You just have to understand that and let your parents sort it out within themselves. But if it's getting really disturbing or becoming an "everyday thing" you can talk to your parents about how it's affecting you and try to convince them to not fight anymore. If that's not working you can always try to spend time away and detach yourself from the situation by spending time at library, playground , at friends house etc. But in any case if it's getting to the point of domestic violence you need to call the emergency numbers immediately. Because that's unacceptable behavior.