Optimised tank hydrodynamics: the key to healthier fish and lower costs

The next generation of recirculating aquaculture systems (RAS) could incorporate “hydrodynamic drive devices” designed to enhance the aggregation and removal of solid waste. With these devices, RAS could achieve more efficient self-cleaning, lower maintenance requirements, improved fish health and, ultimately, higher productivity and profitability.

This proposal, put forward by researchers at Zhejiang Ocean University in a study published in Sustainability, suggests that tank hydrodynamics may hold the key to achieving greater efficiency, healthier fish and stronger financial results in aquaculture.

While RAS are widely recognised for their efficient water use and environmental control, the study draws attention to a factor often underestimated: water flow inside the tank. According to the researchers, “a reasonable tank design can ensure uniform water flow distribution, improve water self-cleaning ability, reduce the probability of disease, and promote the healthy growth of fish.”

An optimal flow not only maintains oxygen and nutrient distribution but also supports self-cleaning by carrying fish waste towards filtration systems. This reduces manual cleaning requirements, lowers stress on fish and cuts labour costs.

The researchers conducted a comparative analysis of circular, rectangular and octagonal tanks. Circular tanks, long favoured for their stable and uniform flow, offer strong self-cleaning properties but inefficient use of space. Rectangular tanks achieve more than 95% space utilisation, yet their design creates “dead zones” where solids accumulate, compromising water quality.

Octagonal tanks are presented as a possible middle ground, combining the spatial efficiency of rectangular tanks with some of the hydrodynamic advantages of circular ones. However, the study notes that “problematic areas can still arise in corners”, which may reduce their effectiveness.

Beyond system efficiency, the study highlights a direct link between hydrodynamics and fish performance. Adequate flow stimulates swimming activity, improves feed utilisation and growth, and strengthens disease resistance. However, flow that is too strong forces fish to expend more energy, while insufficient flow creates stress and slows growth.

As the researchers emphasise, “the relationship between water flow and fish is bidirectional: water flow affects fish swimming, and fish swimming in turn alters water flow.”

By pointing to the potential of “hydrodynamic drive devices”, the authors call for further research into how tank design parameters can be optimised for self-cleaning and welfare outcomes. Such innovations, they suggest, would allow RAS operators to improve production while dedicating less time and resources to maintenance.

The study not only underlines the economic and biological benefits of hydrodynamic optimisation, but also outlines a pathway towards smarter aquaculture systems that could shape the industry in the coming years.

There is no universally “perfect” tank

The study underlines that there is no universally “perfect” tank. Instead, the suitability of circular, rectangular or octagonal systems depends not only on geometry but also on the biological characteristics of the cultured species and even the swimming behaviour and weight of the fish.

For example, highly active swimmers such as salmonids may benefit from stronger, circular flows that stimulate exercise and enhance feed conversion, while species with lower swimming activity or benthic habits perform better in gentler conditions.

The study also highlights that younger fish are more sensitive to excessive flow rates, as they expend more energy when exposed to strong currents, whereas larger, older fish are better able to adapt.

Inadequate hydrodynamics – whether due to excessive or insufficient flow – can lead to slower growth, higher disease susceptibility and greater stress. This makes species- and age-specific adjustments to tank design a critical factor for optimising both welfare and production outcomes.