Probably one of the most ubiquitous and necessary requirements in any aquarium is the proper movement of water within, as well as to and from, the tank. While for many years the main way this was done in tanks was through air stones and lift tubes, most modern aquarium systems rely on electric water pumps of one form or another. Water pumps are typically more powerful, quieter and much more flexible in terms of the applications they can be used for than their earlier air-powered counter parts. The goal of this article, then, is to describe how these water pumps operate, how one goes about selecting a pump for a given application and then how best to install or place your pump for that use.

Centrifugal Pumps

While there are various types of water pumps used in aquariums today, there are really only two which are common. Let's first look at the one which has come to dominate the aquarium industry: the centrifugal pump. Diagram 1 shows the interior of the wet portion of a centrifugal pump (that portion of the pump that makes contact with the fluid being pumped). The pump's operation is as follows: an electrical motor rotates a vane assembly or impeller within the main housing or volute of the pump. The pump housing has both an inlet (where the water comes in) and an outlet (where the water is pumped out); as the impeller rotates, it moves water from the inlet (which is located near the center of rotation of the impeller) along the surfaces of the impeller to the outer portions of the volute by means of centrifugal force (thus, its name centrifugal pump). This water, as it collects in the outer regions of the volute, is directed to the outlet. The water leaving the outlet causes the water pressure to drop at the inlet, which in turn, allows the pump to suck in new water at the inlet to match the rate it is leaving the outlet. The actual flow rate of a pump depends on several factors; most of which will be discussed later, but motor power/speed, impeller design and inlet/outlet sizes all play a significant role.

The other type of water/fluid pump, sometimes used in more complex aquarium systems, is known as a positive displacement pump. These pumps are normally characterized by one of two types: diaphragm pumps or peristaltic pumps. Both of these pumps work on a similar principle - water is first sucked into a chamber and then pushed out that chamber by some form of volume displacement, similar to the way the human heart works. Diagram 2 illustrates the essential parts of a diaphragm pump. Operation of the diaphragm pump starts with the diaphragm being moved by a connecting rod, which leads to a motor/gear system (normally attached to the diaphragm's center). As the diaphragm is pulled out, it creates a low-pressure area that sucks water into it through an input check valve (a check valve only allows water to flow in one direction), thereby filling the chamber created by the displacement of the diaphragm. When the diaphragm movement reaches its limit, it then starts to reverse its direction and starts collapsing the chamber it created; this collapsing of the chamber causes the water in it to be pushed/pumped out a second check valve outlet. As can be seen, a pump of this type is not continuous but delivers its contents (i.e., fluid being pumped) in pulses. By controlling the displacement of the diaphragm and by controlling the rate that it is moved in and out, the effective pump flow rate can be precisely adjusted. That precise control of flow is the reason that these pumps are normally used for applications such as trace element addition and top-off or evaporation replenishment systems.

The peristaltic pump works similar to the diaphragm pump, but substitutes a flexible tube for a diaphragm and pinch rollers for check valves. The flexible tube is routed between a rotor that has two or more pinch rollers on it and a channel that holds and routes the tubing around the rotor (see diagram 3). As the rotor turns, one of the pinch rollers compresses the tubing and pushes any water that may be in it, along in its path toward the outlet. This movement of water created in the flexible tube sucks in new water behind the moving pinched section of tubing (its inlet). As the rotor continues to turn, it eventually brings another set of pinch rollers into play, repeating the operation of the first pinch rollers but with a different volume of water. As with the diaphragm pump, by controlling the rotation rate of the rotor and the diameter of the tubing, you can precisely regulate the flow rate of this pump.

In either centrifugal or positive displacement pumps the materials used in the wet portions of the pumps are critical to their safe use. Whatever materials are used for these wetted parts, they should not contaminate the fluids being pumped nor should the fluids being pumped degrade the materials used in the pump. For aquarium applications this normally means little or no metals that would break down or corrode in water, especially saltwater. For the record, stainless steel will eventually corrode in saltwater and should be avoided when possible, titanium is OK but expensive and brittle. Most wetted parts of the pumps we use are plastic or other non-metallic materials, such as ceramics, that are safe for saltwater. If you are pumping liquids other than saltwater (as in the case of trace element replacement systems) make sure that the fluids you are pumping are safe with the materials used in the pump (outfits such as Cole-Palmer and other chemical equipment houses normally provide tables that indicate what materials are safe together).

As most centrifugal pumps operate via electric motors, you also need some means of isolating the motor from the wetted portions of the pump to prevent the pump fluids from eventually damaging the motor, while at the same time rotating the impeller in order for the pump to work. This connection between motor and wetted portions of the pumps used in aquariums is most often accomplished by magnetically coupling the impeller shaft to a rotating magnet attached to the motor. The impeller shaft has a second magnet attached to it (normally coated to prevent contaminating the fluid being pumped) that is attracted to the rotating motor magnet. By relying on magnetic attraction of the two magnet assemblies it is not necessary to have direct physical contact between the impeller and motor; thus, the two can be sealed from each other. Diagram 4 shows a typical configuration of a magnetically coupled centrifugal pump.

In some centrifugal pumps even the motor portion of the pump is completely sealed, allowing the whole pump to be submersed in the fluid being pumped (power heads being the most common example of this). These "submersible" pumps offer some advantages over their non-submersible counter parts as well as introduce some shortcomings. In an upcoming part of this series on water pumps, we will discuss the installation of pumps in more detail, but it should be fairly obvious that a pump that can be fully submersed is much easier to install. A second advantage is based on the fact that water is a better heat conductor than air (i.e., draws heat away from the pump at a faster rate), so a submersible pump can be made smaller than an equally performing non-submersible pump since heat build up is not a factor in their design. This greater heat transferring property of water also leads to one of submersible pumps potential disadvantages - more heating of the systems water.

Most aquarium systems are fairly easy to heat by use of relatively inexpensive electric heaters. Cooling a tank, on the other hand, is often much more difficult and may require the use of expensive chillers. It is, therefore, highly desirable to control the amount of unintended heating a tank receives. A submersible pump of a given performance will heat a tank more than its non-submersible counterpart. Almost all of a submersible's heat is transferred to the tank; whereas a significant portion of a non-submersible's heat is transferred to the air.

The second possible problem with submersible pumps is the greater danger of electrical shorting to the tank. Most submersible pumps are designed to prevent water from reaching any of the electrical parts, but wear and/or damage to the pump may expose some of these electrical connections, causing dangerous shorts. It is highly recommended that one use GFI (Ground Fault Interrupters) outlets/breakers when using any electrical equipment around water such as pumps, but especially submersible pumps. These GFI devices will detect when electrical shorts occur and immediately shut off the offending pump or piece of equipment, thus reducing the danger to both you and your tank. These two limitations of submersible pumps, namely greater heat transfer and danger of electrical shorting, are likely why you do not often see larger capacity submersible pumps (another potential disadvantage depending on your application requirements).

This concludes the first installment on aquarium water pumps. The next offering will cover how pumps are specified and how this relates to selecting the right pump for your needs or application. A third installment will then follow that will discuss recommended procedures for installing and placing your pumps and some ideas on how to use pumps in various types of marine aquarium systems.