An outside filter is a watertight box suspended outside and alongside of the aquarium. Usually it is made of plastic or glass. The top of the filter is level with the rim of the tank. The filter itself is divided vertically into two unequal compartments by a partition, the lower one-quarter of which is perforated or slotted. The filtering material is placed in the larger compartment. A small tube siphons water from the tank onto the filter material. It sinks through the filter material and runs through the perforations in the partition into the next compartment. The passage of the water through the filter material has cleansed it. The clean water is returned by means of an air-lift tube, which is operated by the pump.

As the intake stem bringing water into the filter is a siphon, it maintains the water in the filter at the same level as the water in the tank. Therefore, the water cannot over-flow. The action is continuous. If the siphoning action is stopped, the return stem - which returns the clear water to the tank - would empty the filter. As the amount of water the filter is usually small compared to the volume of the tank, no harm is done, that is, the tank will not overflow.

For an outside filter to operate efficiently, the water level of the aquarium should be quite high, about one inch from the top. The siphon tube is placed upside down in the aquarium to empty it of air. Still held under water, it is righted, and a finger placed over the shorter open end. Holding this tightly closed, the tube is lifted far enough out of the water so that the short end of the stem can be put into the larger filter compartment, and the longer end remaining submerged in the tank. Only now should your finger be removed from the end. The flow should start and continue until the water level in the filter and the tank is the same.

There are several self-starting stems available today. To start them, hold your finger over the opening in the short end and slowly lower the stem into position. Syringe-type starter balls are also available. This type and the automatic or self-starting stems do offer a good deal of convenience.

Using an Aquarium Siphon

A siphon is a tube, made of rubber or plastic, by means of which water can be drawn out of the aquarium into a container which is at a lower level. A convenient size t use is a 5-foot length of 1-inch inside diameter heavy rubber tubing. Heavy-wall tubing is advisable to prevent kinking and collapsing. There are three methods for starting a siphon:

1. Holding the tube to form a U, put one end under the faucet. When water runs out of the other end, indicating that the tube is full of water, cover both openings with your thumbs. Hold one end underwater in the aquarium and the other end in a bucket lower than the aquarium. Release both ends. Once the flow has started, it will continue automatically.

2. Immerse the entire tube in a container of water, keeping the open ends higher than the center so that all the air runs out of it. Place a thumb over each opening, and then release both ends.

3. The method most frequently used by experts, because it is fastest, should first be practiced with a container of clear water. It involves placing one end of the siphon tube in the water to be siphoned out. The balance of the tube is allowed to hang down. Suck on the lower end until the flow starts, and then quickly drop it into the bucket. A little practice will enable you to tell by the sudden easing in the resistance of the suction pull that the water has come over the top of the loop and is starting down. It will, however, take a little practice so you won’t get a mouthful of water.

An 8- to 10-inch piece of rigid plastic tubing in the end of the siphon hose makes it easier to control and move. The short end in the aquarium is held in the right hand, which guides it over the bottom. The left hand directs the flow into the basin or bucket and regulates the flow, either by pinching the hose or by moving the thumb over the outlet opening.

Stop the flow, hold the suction end about 1 inch over some debris at the bottom of the aquarium, and release the flow momentarily. The debris will be sucked up like magic. By controlling the flow, a maximum of dirt and a minimum of water can be removed. A siphon can be used as a cleansing tool, as a means of emptying the aquarium of water, and as a way of removing gravel - providing the tube is wide enough. In any of these jobs, practice develops control. By putting a funnel at the siphon intake, the siphon will not pick up gravel - only the lighter material.

Surprisingly enough, fishes are seldom sucked up in a siphon. Ordinarily, they avoid it when they feel the suction, although when an aquarium is being emptied, fishes that remain as the water is lowered become panicky and will rush up the siphon tube. The rapid trip rarely injures them so long as they land in a container of water and not on a hard surface like the floor.

Lifting the siphon out of the water without closing the ends will empty the tube and break the suction. Should the siphon tube become clogged and resist efforts to blow it out, place it on the floor and walk along it to crush the clogging material.