There should be a lid on this type of filter and inside the box there should be nylon floss. After you connect the filter to the air supply, the water is filtered through the nylon floss. There are also biological filters. These are the undergravel filters. An undergravel filter is a perforated plate that is under the gravel in the tank, connected to a pump.
Water is drawn through the gravel where the wastes are trapped.
These wastes are then broken down by beneficial bacteria in the tank. A mechanical filter is much more efficient at keeping the tank clean. The biological filters do not work as well and for the most part are considered antiquated. A heater is needed for tropical fish. The rule of thumb to buying a heater is to buy five watts of power per gallon of water.