Environmental Enrichment in Aquariums: What the Studies Show
Evidence on structural, foraging and tactile enrichment for fish from zebrafish, cichlid, salmon and betta studies, with the measured outcomes and the cases where enrichment did not help.
What enrichment means
Environmental enrichment is the stimulation of the brain by its physical and social surroundings. In fish research it usually means adding physical structure to otherwise barren tanks, such as gravel, plants, pipes or caves, or changing how food and other stimuli are presented. A 2026 review of habitat complexity and fish cognition (Klinke & Brown, Animal Cognition) reports that species from structurally complex habitats often have larger brains and larger telencephalon and cerebellum, and that barren hatchery rearing can produce smaller brains, an effect that physical enrichment can mitigate.
Structural complexity and stress
- Zebrafish (Danio rerio): shoals kept for 90 days with a three-way PVC tube showed no rise in water-borne cortisol after transfer to a novel tank, whereas cortisol rose significantly in control shoals; enriched fish also kept greater spacing (Gazzano et al., 2025, Veterinary Sciences).
- Zebrafish: in a judgement-bias study, fish given a sudden gain of structural enrichment interpreted ambiguous stimuli more negatively, and fish with a sudden gain or a gradual loss of enrichment showed more anxiety-like behaviour than fish kept under constant conditions or given enrichment gradually (Buenhombre et al., 2023, Animal Cognition).
- Threespined stickleback, gilthead sea bream and Atlantic salmon: structurally complex habitats reduced aggression in studies summarised by Klinke & Brown (2026); the same review notes studies in sticklebacks, gudgeons, rainbow trout and zebrafish that found no effect of complexity on behavioural variation.
Brain development
In adult male zebrafish kept singly for one week, aquaria with artificial plants and gravel produced more proliferating cells in the telencephalon (PCNA staining) than barren aquaria; whole-body cortisol was generally low but slightly higher in the enriched group, which the authors link to mild stimulation of cell proliferation (von Krogh et al., 2010, Physiology & Behavior). Atlantic salmon (Salmo salar) alevins reared in structurally enriched trays grew larger brains and brain regions than barren-reared alevins, but the difference disappeared after transfer to barren tanks (Näslund et al., cited in Klinke & Brown, 2026). In rainbow trout, physical enrichment increased telencephalic expression of genes linked to neural plasticity, neurogenesis and synaptogenesis (Cardona et al., cited in the same review).
Evidence from cichlids
In Astatotilapia burtoni, where dominant males defend structures as territories, a study housing two males and six females with one, two or three clumped half terracotta pots found that more caves did not increase dominant aggression and did not change mortality, growth, body condition or fin damage in subordinate males; status-related differences in chasing and physiology became smaller as cave number rose (Chamily et al., 2025, bioRxiv preprint, not yet peer reviewed). In the cichlid Geophagus iporangensis, 21 days of tactile stimulation from silicone-bristle sticks increased appeasement behaviour of subordinates during contests with a dominant fish (e Silva et al., 2026, Frontiers in Behavioral Neuroscience). In Neolamprologus pulcher, repeated chasing reduced the number of fish that learned a foraging task, showing that stress affects cognitive performance in this species (Costiuc et al., 2025, Animal Cognition).
Foraging enrichment
Rearing in complex environments has been linked to better foraging. Klinke & Brown (2026) summarise work showing that Atlantic salmon reared in structurally complex rather than barren environments improved associative learning and foraging performance, and that naive juvenile cod raised in complex tanks learned to forage on live prey more efficiently after watching experienced tutors, whereas plain-reared cod did not. Brown, Davidson & Laland (2003) reported that enrichment combined with prior experience of live prey improved foraging in hatchery-reared Atlantic salmon. No fetched study tested a specific food-presentation device for ornamental aquarium fish.
When enrichment backfires
Enrichment effects depend on species. In isolated male Siamese fighting fish (Betta splendens), 22 days of tactile stimulation from silicone bristles did not reduce fighting, increased frontal and lateral displays, raised cortisol and altered brain serotonin; the authors suggest that selection for aggression may make touch a stressor rather than an enrichment in this species (Gauy et al., 2026, Fish Physiology and Biochemistry). Effects on learning also vary: zebrafish reared with physical enrichment showed reduced aversive learning at six months (Manuel et al., cited in Klinke & Brown, 2026).
Practical applications supported by the studies
- Provide physical structure rather than a barren tank: plants, gravel and tube-like shelters reduced stress responses or supported brain cell proliferation in zebrafish.
- Introduce or remove structures gradually: sudden gains and gradual losses of enrichment were linked to anxiety-like behaviour in zebrafish.
- For territorial cichlids, extra caves in a single clump did not increase aggression in A. burtoni, but did not improve subordinate welfare either.
- Do not assume that tactile or novel devices help every species; in Betta splendens bristle stimulation acted as a stressor.
- Keep enrichment in place: brain-size gains in salmon disappeared after fish were moved to barren tanks.
Related topics: stress-in-fish-guide and fish-stress-physiology-cortisol for stress measures, fish-cognition-and-memory-guide for learning, and the territoriality guide for cichlid space use.