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sump
A sump is a separate container connected to the main tank where filtration takes place
What is Protein Skimming?
There are some impurities that accumulate in the water of your aquarium that will most likely not be fully removed via your mechanical and biological filtration. These impurities are called dissolved organic compounds (DOC's). For quite some time, commercial and industrial filtration systems have employed a process called foam fractioning to remove DOC's. A protein skimmer is essentially a foam fractioning system for your aquarium. It pulls out water, removes DOC's, and returns the clean water to the tank. If not removed from the water, the DOC's would result in a buildup of oils and phosphates from fats, and nitrates from proteins. Exactly how protein skimmers remove DOC's will be explained below.

How Does a Protein Skimmer Work?
Foam fractioning is a relatively simple process. If you agitate pure water by blowing bubbles into it, the bubbles at the surface burst quickly and no foam develops. However, if the water has fats and/or proteins dissolved in it, the bubbles will not burst as easily and a foam will form as you continue to inject air bubbles. If you've ever blown bubbles with a straw in a glass of milk, you've seen this in action. Milk is basically water with some fats, proteins and sugars dissolved in it. So when you blow bubbles in it, a foam forms. If you were to continually blow bubbles into the milk and scoop the foam off the top, you would eventually remove most of the DOC's (fats and proteins), as well as some of the water.
This is what a protein skimmer does. It creates bubbles in the water and as foam forms on top of the water, it falls into a collection container while the clean water (the stuff that wasn't part of the foam) is returned to your tank. There is a variety of mechanisms by which different protein skimmers accomplish this, but they all operate on this basic idea.
Do I Really Need a Protein Skimmer?
Although protein skimmers are pretty simple conceptually, they can get a bit pricey. When setting up a new aquarium, there are many different accessories that can be added and most people don't want to spend the money to get every possible add-on right off the bat. So, naturally, you want to know just how essential each accessory is.
Protein skimming is not absolutely essential, but it is close. You will be able to keep fish alive in your tank without it, but it certainly won't be an ideal environment. It's kind of like getting a hamster, but not cleaning its cage. It will live for a good while, but it will be constantly immersed in its own waste and that will gradually take its toll. If you can't afford one right away, consider waiting a little longer and saving up a little extra money to get yourself a protein skimmer. Once you have it running and you see the brown scum that it collects from your water, you'll understand why you needed it