Abstract

This article explores how environmental enrichment and training practices can be effectively applied to improve the welfare of aquarium fish. Drawing on scientific research and insights from leading animal welfare organizations, the paper emphasizes the importance of individualized, goal-oriented strategies that recognize species-specific needs and behavioral diversity. It argues that understanding and meeting the specific needs of each fish — through careful assessment and preference testing — can significantly improve their well-being. Given that captive animals often have limited agency, the article highlights the critical role of agency in enrichment, advocating for approaches that allow animals to make choices and engage in species-typical behaviors. The paper explores key welfare models, enrichment frameworks, and assessment tools, highlighting the role of preference testing and behavioral observation in tailoring care. To conclude, the article provides practical strategies for creating complex, stimulating, and responsive habitats. By adopting these practices, we can create environments that support both the physical and psychological welfare of pet fish, leading to a more ethical and fulfilling captive experience.

Introduction

Enrichment and training are increasingly recognized as essential components of animal welfare, yet their application to aquarium fish has lagged behind that of mammals and birds. While fish are often perceived as passive or low-maintenance pets, growing scientific evidence reveals their behavioural complexity, cognitive abilities, and capacity for suffering and pleasure. This article explores how enrichment and training can be thoughtfully applied to improve the lives of pet fish in home aquariums. Drawing from animal welfare science and practical examples, it offers guidance on how to create environments that meet species-specific and individual needs, promote behavioural diversity, and support agency. The goal is to move beyond minimal standards of care and toward practices that foster meaningful engagement and well-being for captive fish.

Welfare

Animal welfare encompasses more than the absence of suffering; it includes the full scope of an animal’s physical health and psychological well-being. The International Fund for Animal Welfare (IFAW) defines good welfare as a state that supports both safety and positive emotional experiences – what they term “a life worth living” (IFAW, n.d.).

For aquarium fish, good welfare depends on caregivers providing conditions that allow for species-specific behaviours, appropriate husbandry, and positive social environments (Smith, 2023). However, these measures are only effective when they are guided by ongoing assessment. Enrichment, for example, must be evaluated based on whether it actually supports the animal’s physical and psychological needs, rather than simply being present.

Tools like the Five Domains Model (Mellor et al., 2015) and the MYFishCheck framework (Tschirren et al., 2021) provide structured approaches to evaluating welfare. MYFishCheck, although developed for aquaculture, outlines essential welfare needs — ranging from homeostasis and nutrition to environmental safety and social interaction — that are broadly applicable (see Table 1). Still, the diversity of species kept in home aquariums means no universal checklist can be applied; welfare assessments must be tailored to individual species and contexts.

Table 1. Fish welfare needs, adapted from MyFishCheck 2021

While some indicators of poor welfare, such as bloating, lethargy, or inappetence, may be apparent (AVMA, n.d.), the absence of such signs does not imply well-being. As Miller (2024) argues, positive welfare is better assessed through behavioural diversity — the range of species-typical behaviours animals display when their needs are met and they have meaningful control over their environment. Enrichment should therefore offer opportunities for self-maintenance, choice, control, and cognitive stimulation. In short, good welfare is not only about preventing suffering, but about enabling a life in which fish can engage, explore, and thrive.

Understanding welfare provides the foundation for meaningful enrichment. Enrichment is not an optional add-on to care — it is a key tool for achieving and maintaining good welfare. When designed thoughtfully, enrichment supports the expression of species-typical behaviours, fosters agency and choice, and addresses both the physical and psychological needs of the individual. In this way, enrichment becomes a direct and practical expression of our welfare goals, bridging the gap between theoretical understanding and lived experience for the animals in our care.

Enrichment

Enrichment plays a crucial role in ensuring the well-being of animals living in human-built environments, where their ability to engage in natural behaviors is often restricted. Bender (2023) describes enrichment as the process of meeting an animal’s physical, behavioral, and emotional needs in ways that allow them to express species-typical behaviors safely and appropriately. This is especially important for animals in captivity, as they lack access to natural environments and the autonomy to seek out experiences that fulfill their needs. Without intentional and well-designed enrichment strategies, their opportunities for choice and control remain severely limited.

Natural history

Designing effective enrichment for fish requires careful consideration of their natural behaviors, environmental needs, and individual differences. Rather than taking a one-size-fits-all approach, we should ask key questions to ensure that enrichment provides meaningful benefits. The following table outlines critical questions to guide this process.

A list of resources is provided at the end of the article for research purposes.

Individual needs

Enrichment is not a one-size-fits-all solution. As The Shape of Enrichment, Inc. (n.d.) emphasizes, “when considering animal welfare and environmental enrichment, it is critical to remember that both are individual-based. … [W]hat is beneficial to one individual may not be for another.”

For fish, individual differences such as species, life stage, health status, and prior experiences all influence what types of enrichment are effective (Nӓslund & Johnsson, 2014). Some fish may benefit from increased hiding spaces to reduce stress, while others may require more complex environments that encourage active foraging or exploration. Understanding these individual needs ensures that enrichment is not just an aesthetic addition to an enclosure but a meaningful improvement in welfare.

People who haven’t kept fish may be surprised to learn that they have different temperaments. Temperament is defined as an individual’s consistent pattern of reactions. Animal behavior studies typically focus on the shyness-boldness continuum (APA, 2024). As with other animals, there are species generalizations that serve as a starting point and there are individual variations and exceptions.

Aquarium stores typically categorize fish as “peaceful,” “semi-aggressive,” and “aggressive.” This gross oversimplification creates inappropriate expectations. For example, ropefish (Erpetoichthys calabaricus) are often labeled as “aggressive” because they are large carnivores. (As behavior professionals, we recognize that the word “aggressive” is not a temperament, but rather a description of behavior in certain contexts.) But ropefish aren’t active hunters; they are bottom-feeders. When they aren’t swimming or foraging, they can often be found resting, piled on top of each other. While it would not be wise to house bite-sized fish with them, they are by no means aggressive.

Figure 1. Ropefish (Erpetoichthys calabaricus) resting together.

On the other hand, guppies (Poecilia reticulata) and swordtails (Xiphophorus hellerii) are considered “peaceful.” Yet, it’s not uncommon for them to kill each other, either by fighting or nonstop mating. Not exactly “peaceful.” It is important to research species ahead of time and also to recognize that individual fish have different temperaments and may not behave as expected.

Types of enrichment

Enrichment is a broad concept that encompasses multiple dimensions of an animal’s well-being. Various researchers and organizations have categorized enrichment in different ways, highlighting its complexity and the need for a tailored approach.

The Shape of Enrichment, Inc. (n.d.) identifies five overarching categories of enrichment: physical habitat, food, sensory, cognitive, and social. These categories emphasize the importance of addressing both environmental and behavioral needs.

Expanding on this, Bender (2023) proposes a more detailed framework with 14 categories, including: health and veterinary care, hygiene, nutrition, physical exercise, sensory stimulation, safety, security, species-typical behaviors, foraging, social interaction, mental stimulation, calming, independence, and environment. This broader view acknowledges the interconnected factors that contribute to an animal’s ability to engage in natural behaviors.

Smith (2023) offers a more practical approach by outlining key considerations for fish welfare in captive environments:

• Water quality
• Species selection
• Nutrition
• Habitat design
• Enrichment

While these frameworks vary in scope, they collectively reinforce the idea that enrichment must go beyond simple additions to an enclosure. Instead, it should be an integrated part of an animal’s care, designed to meet both species-specific and individual needs:

Table 2. Enrichment strategies for aquarium fish.

In an aquarium, the majority of enrichment is environmental. A complex environment is the most important way to promote behavioral diversity (Clegg, 2023). A meta-analysis by Zhang et al. (2021) found that aquatic animals from physically enriched environments had significantly improved welfare, compared with their counterparts from barren environments.

This highlights the critical role that environmental complexity plays in promoting both physical and behavioral health for captive aquatic animals. By offering a variety of stimuli, such as varied substrates, hiding spots, and objects that encourage exploration, aquarists can foster a more dynamic and fulfilling habitat. However, environmental enrichment alone may not be enough; it should be complemented with other forms of enrichment, including sensory and social aspects, to address the full spectrum of the animal’s needs. The goal is not just to prevent harm but to actively enhance the quality of life for the animals in our care, supporting their natural behaviors and providing opportunities for choice, stimulation, and well-being.

Enrichment doesn’t have to look natural to meet an animal’s needs. For example, plecostomus are bottom-dwelling catfish. In the wild, they hide in caves formed by logs and rocks. In an aquarium, they will use terra cotta pots and PVC pipes.

Figure 2. Plecostomus (P. sabaji) using a PVC pipe cave.

Goals and Assessment

Enrichment should be goal-oriented, meaning there should be specific, measurable outcomes so we have evidence of improved welfare (Tschirren et al., 2021). Simply adding a structure or providing a puzzle feeder is not sufficient to enhance a fish’s quality of life (Nӓslund and Johnsson, 2014). Effective enrichment must be tailored to the individual’s needs, preferences, and cognitive abilities to have a meaningful impact on welfare. These efforts must take into account the individuals and the community as a whole (all of the individuals in the aquarium).

Goals can be specific or general. For example, a general goal could be to reduce stress, while a specific goal could be to be able to clean the aquarium without the fish darting around frantically.

Take, for example, the goal of reducing stress. Consider how the fish react when you walk by:

If the fish exhibit avoidance behavior by darting away, consider adding real or artificial plants to establish visual barriers, providing them with a greater sense of security. Additionally, moving past the tank at a slower pace may help reduce stress — though this should not require excessive caution in your own home. In breeding tanks, placing a curtain made of paper or fabric over the front of the tank can further enhance their sense of safety.

Figure 3. Breeding tank for angelfish (P. scalare) with a curtain for security.

If the fish approach you, they have probably learned to associate your presence with food. Try to feed on a consistent schedule to reduce frustration while avoiding overfeeding.

Figure 4. Freshwater angelfish (Pterophyllum scalare) at feeding time.

Once an enrichment plan has been created and strategies are implemented, it is important to periodically assess whether the enrichment strategies are (still) meeting the goals (Mackie, 2023). Because animals need variety in their lives, it’s not unusual for a strategy that was working previously to become less effective over time, as is seen when an animal habituates to a puzzle feeder.

It is important to be flexible in your enrichment approaches and to consistently assess the individual’s needs. In a community tank, the needs of each species, as well as each individual animal, must be considered. For example, in an aquarium including Corydoras catfish, plecostomus (suckermouth catfish), and angelfish — all species that come from South American rivers — feeding presents a challenge. The catfish are bottom-feeders; they favor dense, sinking foods. Angelfish are assertive feeders, often consuming most of the food before it can reach the bottom. Because angelfish are cichlids, they are particularly susceptible to bloat, a condition in which fluid accumulates in the body cavity, putting pressure on the organs. If they consume food that is too dense for them to digest, it can be fatal. My goal in this situation was to allow the corys to eat their preferred foods without endangering the angelfish, which required some innovative thinking. We designed and 3D printed a dome that would allow the catfish to travel in and out, but the angelfish could not get inside. We called it the “Cory Dome”.

Figure 5. The Cory Dome with Corydoras catfish and adult angelfish too large to get inside.

The “Cory Dome” worked for a while but, when new juvenile angelfish were introduced into the community tank, I was concerned that some of them were small enough that they might be able to swim into the openings and get stuck. A friend suggested using a pepper grinder to break the food into smaller pieces. This ensured that, while the angelfish might consume a few bites, they wouldn’t eat enough to bloat, and they would not be able to consume all of the food before the catfish could access it.

Figure 6. Fish food chips in a pepper grinder.

Later, I tried out different foods and found that the catfish enthusiastically eat some frozen foods that don’t cause a problem for the angelfish, so I no longer use dense, sinking food in those community tanks.

Preference tests

Clegg (2023) raises an essential question for those working with captive animals: “Do we know what the animal wants? Do they have the opportunity to make a choice? (As in, a positive choice between things they want to do, not a choice between negative outcomes.)” This highlights the importance of understanding not just what animals need for their survival, but what they desire for their well-being and emotional fulfillment. Preference tests are one way to assess animals’ choices in a controlled environment, offering them a selection of options and observing which ones they prefer. These tests provide valuable insight into the natural preferences of the animal, such as social interactions, food types, or types of environmental enrichment. By understanding these preferences, we can create more personalized, effective enrichment strategies that align more closely with the individual needs and desires of the animals in our care.

One way to better understand what animals want is to look at time budgets of what the species would be doing in the wild and make a comparison. If that information is not available, we can set up preference tests and record space use over 24 hours. It is important to include 24 hours because animals often behave differently when humans aren’t around. This is particularly important for crepuscular and nocturnal species (Clegg, 2023).

There has been very little research on welfare best practices and the environmental preferences of ornamental aquarium fish. Most research has focused on raising salmonids in aquaculture, or using zebrafish in laboratory experiments. Research conducted for zoos and public display aquariums is often inaccessible to a general audience, hidden behind a paywall. One study by Sullivan et al. (2016) found that goldfish preferred plants over empty space, but they did not discriminate by type of plant.

To better understand how we can optimize environmental enrichment for individual fish, preference tests offer a useful approach. These tests allow us to assess what specific conditions contribute to an animal’s comfort and well-being.

Case Study: Preference Tests for Individual Fish Welfare

I conducted a series of preference tests for my betta (Betta splendens), Alastor, to determine which environmental conditions make him most comfortable. The plan was to change one thing (substrate, decor, background, light, etc.) on either the left or right side of the tank, record data using a trail camera over a period of at least 72 hours, and then switch the conditions on the left and right side.

Through this experiment, I was able to observe his behavior in different settings and gain valuable insights into which factors -— such as water temperature, habitat complexity, and light levels — promoted his comfort and overall welfare.

First, I gathered some baseline data. I was surprised to learn that, all conditions being equal, Alastor showed a preference for the left side of his tank by spending the majority of his time there (77%). Try as I might, I was unable to figure out why! There was no obvious difference between the left and right side of the tank. Perhaps something about the external environment (the lighting in the room, the direction people approached from, etc.) made a difference.

From May through August, I conducted experiments with sand and gravel substrate, and real and artificial plants, to determine whether Alastor’s preference for one side of the tank over the other was independent from the conditions on the left and right side. Alastor showed a preference for gravel substrate, spending 75% of his time there, enough to override his preference for the left side of the tank. He did not show a preference between real and artificial plants. After discovering that Alastor prefers gravel, I spread out the gravel across the bottom of the whole tank.

Over the summer, I paused the experiments to focus on breeding angelfish. During that time, I did not move anything in Alastor’s tank (except minimal movement while doing water changes). At some point, he switched his preference from the left side to the right side of his tank. Now, he spends most of his time resting in the fake plant on the right side of the tank.

Since then, I have done two experiments: one with different colored backgrounds and one with light. Alastor appeared to choose the white background more often, but I suspect that he is just easier to observe when photographed against a white background. He had a strong preference for the right side of the tank, regardless of whether there was additional light on that side.

Figure 7. Preference testing setup, comparing black and white background.

Of course, these results are for one individual fish, in a specific setting. If I were to move Alastor’s tank to a different room of the house, or transfer him to a different tank, or if I were to experiment with another betta, I would get different results. The point of preference tests is not to be able to generalize to the population, but to figure out what the individual animal needs to have the best welfare in that context.

In the fall, Alastor started looking bigger and healthier than before. His body now shines with spots of blue iridescence, and his crowntail fins have grown long and developed blue-white tips. I have never seen a betta look so good!

Recently, I added floating plants to Alastor’s tank and he started building a bubble nest for breeding, which is an indicator of good welfare. After several weeks of building bubble nests on the right side of his tank and not finding a mate (I have no desire to breed bettas!), Alastor has switched sides again. Now, any time I walk past the tank, I nearly always find him on the left side. He spends more than 90% of his time there.

Figure 8. Alastor (B. splendens).

The key takeaway for me from all this preference testing is that it takes time for fish to settle into a new environment. Alastor, at least, does better when he is left undisturbed, without the conditions in his tank changing constantly.

Conclusion: Tailoring Enrichment to Individual Needs

Enrichment is not a one-size-fits-all approach; it must be tailored to meet the individual needs of each animal, particularly in captivity. For animals in an aquarium setting, where environmental enrichment is the most feasible form, it’s critical to consider both the physical and psychological aspects of their welfare. Enrichment strategies must be goal-oriented, ensuring that they promote species-typical behaviors, foster physical and mental health, and support overall well-being.

However, enrichment should not be static. Regular assessment of enrichment strategies is essential, allowing for adjustments based on the evolving needs and preferences of the animals. This iterative process is key to ensuring that interventions remain effective in promoting welfare.

The use of preference testing serves as a valuable tool in this effort. By providing insight into what an individual animal truly wants and needs, this testing empowers them to make choices that enhance their autonomy and agency — elements that are often lacking in captive environments. Ultimately, by focusing on the unique preferences and well-being of each animal, we can create more enriching, fulfilling lives for those in our care.

References

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Bearly, I. (2019, December 24). How to pick the best substrate for a freshwater planted aquarium. Aquarium Co-op. Retrieved March 21, 2024, from https://www.aquariumcoop.com/blogs/aquarium/planted-aquarium-substrate

Bender, A. (2023, July 29). The 14 categories of pet enrichment. Pet Harmony. Retrieved March 20, 2024, from https://petharmonytraining.com/the-14-categories-of-pet-enrichment/

Clegg, I. Animal Welfare Expertise. (2023, December 5-8). Why providing choice and control opportunities is critical for good animal welfare [Conference presentation]. ZooSpensefull 2023 Cooperative Care Conference, virtual. https://zoospensefull.com/courses/cooperative-care-conference-2023/

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Additional Resources

Seriously Fish (https://seriouslyfish.com/)

Planet Catfish (https://www.planetcatfish.com/)

Misty Hampton, CCBC, FFCP, is an aquarist, as well as a cat trainer and behavior consultant. She is the owner of Fellow Creatures Pet Consulting. Misty started teaching at age 12. For 13 years, she was an instructor for a home-schooling community. Since 2001, she has been a math and science tutor. Now, she runs a birding club for Lake Oswego Parks & Recreation.

Nature and animals have always been in Misty’s life. She worked at Cat Adoption Team for two years. Then, she spent a year working at a freshwater aquarium warehouse, where she became obsessed with catfish and also was inspired to start breeding angelfish. Her passion is helping people better understand misunderstood animals. She shares her life with her husband Jesse; their five cats: Linus, Jasper, Abigail, Pearl, and Piper; and a bunch of fish — too many to count!