How has no one hopped on this post yet?! The purpose of a protein skimmer is basically the most crucial piece of equipment most of us will say that creates bubbles in a chamber, those bubbles rise and with those rising bubbles organic compounds such as waste and food particles will be pushed towards the top and into a collection cup. It removes waste before the waste has a chance to break down and turn into nitrates and phosphates. It helps especially if you have a lot of fish, and have trouble keeping nitrates and phosphates down. There are those who don't use them and it's not like if you don't use a skimmer everything will die no it's just a very nice piece of equipment because it makes all of our lives easier. It's a great way to remove waste, it's efficient at it, doesn't require media changes like changing out poly or floss pads whatever you wanna call them. I guarantee you everyone will have their own opinion about this topic but here's the deal. If you have a tank smaller than 20-30 gallons and you maintain your tank and your not overstocked with fish, chances are you won't even need one because one 20% water change a week or every two weeks will basically add up to a skimmer. Now their are skimmers for those smaller scenarios and they are relatively cheaper anywhere from $25-100 where bigger tanks like 55+ gallons will run anywhere from $120+ you get the idea. But here's a video on it.