Microscopic particles in a fish tank with a healthy rainbow trout swimming through the water

The Hidden Dangers in Your Fish Tank: Micro Particles and Your Trout's Health

Elliot Brynn in Science & Nature June 2025 • 4 min read.

"Uncover the surprising link between tiny particles, bacterial activity, and the well-being of rainbow trout in aquaculture. Learn how to maintain healthier fish and a cleaner system."

Recirculating aquaculture systems (RAS) are innovative setups designed to maximize water reuse, but this intensity can lead to an unintended consequence: the build-up of micro particles. These particles, smaller than 20 micrometers, come from various sources, including fish waste, uneaten feed, and sloughed-off material from biofilters. While these systems have filtration units, micro particles are often too small to be effectively removed, leading to their accumulation over time.

The problem with accumulating micro particles is their potential to negatively impact water quality. Suspended organic solids act as a feast for bacteria, driving up the biochemical oxygen demand (BOD) and carbon dioxide levels in the water. Some even speculate that these particles can cause physical harm to fish gills. It's a complex issue that demands a closer look.

Now, a groundbreaking study sheds light on the real-world conditions of micro particle accumulation in Danish rainbow trout farms. Unlike previous studies conducted in controlled, experimental settings, this research investigates commercial-scale operations to understand the true scope of the issue.

The Microscopic Culprits: Understanding Micro Particle Dynamics

Microscopic particles in a fish tank with a healthy rainbow trout swimming through the water

The study, conducted across seven commercial rainbow trout farms in Denmark, analyzed water samples from twenty separate RAS units. The aim was to measure the levels of micro particles (numbers, volume, and surface area) and bacterial activity, alongside key water quality indicators. This comprehensive approach allowed researchers to paint a detailed picture of the factors at play in these systems.

The results were striking: micro particle numbers varied wildly, ranging from 6.0 x 10^4 to 7.4 x 10^5 particles per milliliter. What’s more, there was a strong positive correlation between micro particle levels and bacterial activity. This confirms that these tiny particles serve as hotspots for bacterial growth within the RAS environment.

Here are some of the key findings:

High Variability: Even seemingly similar RAS units within the same farm showed drastically different levels of micro particles.
Bacterial Hotspots: Micro particles provide ample surface area for bacteria to colonize, leading to increased bacterial activity.
Filter Effects: Biofilters tend to trap particles, while drum filters seem to reduce particle volume but increase particle numbers and surface area.
Commercial Scale: The study provides a crucial baseline for micro particle levels in real-world trout farms.

While biofilters generally trapped particles, drum filters seemed to have a different effect. They reduced particle volume while paradoxically increasing particle numbers and surface area. This suggests that drum filters might be breaking down larger particles into smaller ones, increasing the overall surface area available for bacteria to colonize.

Implications for Sustainable Aquaculture

This study underscores the need for careful management of micro particle levels in recirculating aquaculture systems. By understanding the dynamics of these particles and their relationship with bacterial activity, fish farmers can implement strategies to improve water quality and promote the health and well-being of their trout. Further research is needed to determine optimal particle levels and effective methods for their control, ensuring the long-term sustainability of aquaculture practices.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1016/j.aquaeng.2018.12.001, Alternate LINK

Title: Micro Particles And Microbial Activity In Danish Recirculating Rainbow Trout (Oncorhynchus Mykiss) Farms

Subject: Aquatic Science

Journal: Aquacultural Engineering

Publisher: Elsevier BV

Authors: Kim João De Jesus Gregersen, Per Bovbjerg Pedersen, Lars-Flemming Pedersen, Johanne Dalsgaard

Published: 2019-02-01

Everything You Need To Know

Where do micro particles in recirculating aquaculture systems come from, and why do they accumulate?

In recirculating aquaculture systems, micro particles accumulate from fish waste, uneaten feed, and sloughed-off material from biofilters. While filtration systems are in place, these micro particles, being smaller than 20 micrometers, are often not effectively removed. This leads to a buildup over time, creating an environment where water quality can degrade, as the particles become a food source for bacteria, increasing biochemical oxygen demand and carbon dioxide levels.

What range of micro particle levels did the Danish rainbow trout farm study reveal, and what did it demonstrate about bacterial activity?

The study revealed a wide range of micro particle concentrations across different RAS units, from 6.0 x 10^4 to 7.4 x 10^5 particles per milliliter. This variability existed even among seemingly similar units within the same farm. Additionally, the research confirmed a strong link between the levels of micro particles and bacterial activity, indicating that these particles act as hotspots for bacterial growth.

How do biofilters and drum filters affect micro particles differently in recirculating aquaculture systems?

Biofilters tend to trap micro particles, reducing their overall presence in the system. Drum filters, on the other hand, appear to reduce the volume of particles but paradoxically increase the number of particles and their surface area. This may be because drum filters break down larger particles into smaller ones. This is important because the increased surface area allows bacteria to colonize.

What are the specific effects of micro particle accumulation on water quality and the health of rainbow trout?

The accumulation of micro particles can significantly impact water quality by increasing bacterial activity, which in turn raises the biochemical oxygen demand and carbon dioxide levels. It has been speculated that high concentrations of micro particles can cause physical harm to the gills. The study highlights the need for better management of these particles to maintain water quality and the health of the fish. More research is needed to determine the optimal levels of micro particles.

Why is it important to manage micro particle levels in recirculating aquaculture systems for long-term sustainability?

Managing micro particle levels is crucial for the long-term sustainability of aquaculture practices. Understanding the dynamics of micro particles and their relationship with bacterial activity allows fish farmers to implement strategies that improve water quality, promote fish health, and ensure sustainable production. Effective methods for controlling micro particles need to be developed through ongoing research. This ensures the health and sustainability of trout farming.