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Water flow should be suitable for the species and provide adequate exposure to the components of the LSS. Appropriate water flow (velocity and direction) directly impacts animal health and welfare:

 Promotes gas exchange at the surface and prevents anoxic or stagnant pockets of water.

 Promotes healthy‐system bacteria in biofilms and reduces fouling bacteria in the sediment.

 Helps transfer nutrients and waste products and reduces sediment buildup.

 Reduces turbidity in the water column.

 Prevents thermal stratifications (layers) in the water column.

 Ensures correct flow through the LSS.

 Reduces pathogen loads, particularly ectoparasites.

 Affects animal behavior and feeding; these behavioral effects can be used to improve visibility of fish in display aquariums.

Water flow is typically achieved using water or air pumps. The most common is a centrifugal water pump (Figure A3.2). This is a very simple machine that imparts energy to the water by “slinging” water out of its impeller with centrifugal force, resulting in flow. They come in a variety of types and sizes. Power is reported as flow per unit time (e.g. gallons per minute). It is important to note that pumps produce noise, vibration, and heat. The effects of these on fish systems should be mitigated where possible (e.g. rubber mounts for pumps).

Figure A3.1 Gas exchange using an air stone (a) and a degassing tower (b).

Source: Image (a) courtesy of Catherine Hadfield.

When designing an LSS, pump size is a critical consideration. Pumps affect the rate and pressure at which water moves through the individual LSS components, controlling their efficacy and efficiency. LSS components often have flow rate and pressure specifications they must operate under to perform adequately. The desired rate of flow is also dependent on the size of the LSS components relative to the amount of organic material (e.g. animal waste, uneaten food, detritus). Exposed outdoor systems typically have a water flow and LSS that is twice the size of an indoor system of similar size and population to help deal with external inputs such as leaves and other debris.

Figure A3.2 Centrifugal pumps for water flow. There are mechanical filters (known as basket filters) upstream of the pumps to reduce the risk of debris getting into the pump and ball valves to control water flow.

Source: Images courtesy of Catherine Hadfield, Seattle Aquarium.

In some cases, all the water will go through life support equipment (batch‐treatment). In other cases, some water may be diverted through life support equipment (side‐stream), e.g. passing a percentage of the water through ozone disinfection.

The turnover rate is the time the total water volume of the system might take to go through the LSS; this has a profound impact on water quality. Theoretically, an aquarium with a system volume of 6300 gallons flowing through a canister filter at 30 gallons/min has a turnover rate of ~3.5 hours. However, not all water in the system will be treated in the 3.5 hours, as the water mixes in the enclosure itself. So the actual water going from the main enclosure to the LSS filters is always a mixture of different filtration histories. This mixing is described by Gage and Bidwell's Law of Dilution. Gage and Bidwell conducted experiments on how many turnovers it actually takes to filter all of the water in a pool using calibrated soil additions and measurements of turbidity. They determined that only ~40% of a pool's water actually gets filtered in one so‐called turnover, and that it takes about four turnovers to filter 98% of the water. As a result, achieving four turnovers in a day or a six‐hour turnover rate became the accepted norm for large swimming pools. More than 30 years ago when large aquarium systems became more common, LSS engineers reasoned that a higher turnover rate was likely needed for living systems and modern systems are typically designed with turnover rates of an hour or less.

There are a variety of flowmeters on the market that can be used to monitor critical flow rates. Common types are mechanical, magnetic, and ultrasonic. Mechanical flowmeters measure the speed of rotation of an impeller or paddle wheel within a pipe. They are the simplest and least expensive, but are less accurate than the other two and the internal parts are vulnerable to damage or obstruction. Magnetic flowmeters work according to Faraday's formula, where flow of a conductive liquid medium (the water) is proportional to the strength of a magnetic force passing through it, measured in volts. Magnetic flowmeters come in both insertion probe or in‐line styles that can be mounted anywhere, and one‐ or two‐beam (for better accuracy) types. They are low‐maintenance, show better accuracy, provide minimal to no obstruction to water flow, and have no moving parts, but are more expensive. Finally, strap‐on ultrasonic flowmeters bounce high‐frequency sound off solids and bubbles in the water to create a frequency shift that is directly proportional to flow rate (Doppler type), or use two bursts of sound, one in the direction of water flow (which moves faster) and one against it (which moves slower) (transit time type) to calculate flow rate. They are very easy to use, very accurate, highly portable, and can work on a wider range of water types, since they do not work off of the conductivity of the water, but they do need a run of pipe equal to or more than 10 pipe diameters ahead of them and 5 after and are the most expensive.