Once you have water, rock, and sand in your tank, you will need to have the water flowing through the tank to provide oxygen for the fish to breath and clean the tank out. A filter is how this is done.

A filter takes the water in a circle in your tank. This helps to clean out the fish and reef creature waste.

There are a few kinds of filters that can be bought. A regular filter for a fresh water tank can not be used as it doesn't have the right sections to clean out the extra waste that marine tanks have.

One such filter is a multi-chamber filter. They can be bought 2-4 chambers. More is better as there are different medias to use. Charcoal is one such media. This removes impurities and makes the water better for the fish. Fluval is one type of filter that comes as a multi-chamber although there are others. They are also expensive if bought new. But once bought all that needs to be purchased is the media. And depending on the media type can be changed every 2-6 months. Some only needs to be cleaned every month and then replaced every 6-12 months. Sponges, which remove the most impurities, need to be replaced monthly, but they are also the least expensive item to buy.

Another kind of filter is a refugium. This is a tank with multiple chambers as a filter. There will be bio-balls, to collect big waste. They need to be cleaned every few months, although only half at a time because they also collect good waste for the tank. This moves into a tank like area where you put sand again. Plants and food can be stored here. The plants help by "eating" the waste of the fish and producing helpful bacteria for the fish tank.

Sand shifting creatures can be kept in this part as well and help keep any sand in the refugium healthy.

The water flows into a small portion that has a sponge in it to catch the big particles that are not wanted in the tank again. It needs to be replaced monthly. It flows through the sponge to a chamber that takes the water back to the tank.

For it to go to the refugium it goes through an overflow box. This helps keep your tank creatures from getting sucked into your refugium, which could kill them. The overflow takes the water down to the bio-balls.

Refugiums can be made or bought. Store made ones are usually called wet/dry filters but are very similar in style.

A pump will be needed to run the refugium. These can be expensive if bought in a store but places like Ebay have them in abundance for much cheaper prices, even with shipping. A multi-chamber filter will have the pump built in.

The filter will be set up and started after the water is put in the tank. A little water should left out so that it doesn't overflow from the displacement of the filter parts.

Once the filter is in and working the tank is set up. And then light needs to be added.