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.

What is the Best Location for a Tropical Fish Aquarium Pump?

It is usually advisable to have the pump at a higher level than the tank. This will prevent any possibility of a siphoning action draining the water from the tank back into the pump. Pump platforms are available for pump placement. These are small plastic or metal shelves designed to hold the pump while hanging on the back of a tank.

If it is desirable to place the pump on the floor, or on any level below the level of the tank, there are several precautions to be observed: Backflow seldom starts while the pump is in operation; it is when the pump is not operating that the greatest danger occurs. Backflow may be started by several actions.

(1) The tendency of water to rise in a narrow tube is known as capillary action. If the water in the tank is very close to the top, it is possible for water to rise in the air tube by capillary action to a point where it starts a downward flow and a siphoning action occurs.

(2) The cooling down of a warm pump can cause the air inside to contract and draw the water down.

(3) Over- oiling the pump can cause excess oil to enter the lines. Air pressure will keep the oil clinging to the inside walls of the vertical tubing, but with the air pressure removed; the oil starts to run back. As it collects in droplets, a vacuum is created behind it strong enough to start a siphoning action.

In order to prevent this, keep a small surgical type “C” clamp handy. When, for any reason, the pump is shut off, clamp shut the line leading to the pump. Also shut all air valves tightly. Whenever oil or moisture is seen in a tube, discard that piece of tubing. It is also a good idea to clean the pump when excess oil appears in the line.