Fish "anti-stress": the only measured lever runs through the food
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
The aquarium aisle offers "anti-stress" bottles to pour into the water after a move or a water change. The scientific literature does not test those products: it tests an amino acid added to the food. Our feeding guidance is gathered in the fish food section.
Contents · 5 sections
Cortisol cut threefold in tilapia
Mohammady, Kamal, Elashry, El-Sayed and Hassaan published in 2025, in Scientific Reports, an open-access study on dietary L-tryptophan in Nile tilapia fingerlings exposed to ammonia (see the study). The protocol compares four doses — 0, 2, 4 and 8 g per kilo of feed — over 70 days, followed by 10 days of exposure to 5 mg/L of total ammonia nitrogen. In exposed fish, serum cortisol falls from 10.86 ng/mL without tryptophan to 3.60 ng/mL at the 8 g/kg dose. Without exposure the drop also occurs, from 6.19 to 2.29 ng/mL.
A comparable result was obtained in an ornamental species: Morandini and colleagues published in 2015, in General and Comparative Endocrinology, a study on a South American cichlid fed an enriched diet for four weeks. The fish showed higher forebrain serotonergic activity and a significant reduction in cortisol, with no effect on sex steroids or growth.
Two caveats that rule out calling it a sedative
Lepage, Tottmar and Winberg published in 2002, in the Journal of Experimental Biology, a study on rainbow trout fed for one week on pellets enriched to 2, 4 or 8 times the commercial concentration. In fish subjected to a stressor — a two-hour drop in water level — the cortisol rise was indeed blunted. But in undisturbed fish, tryptophan slightly raised cortisol. It is therefore not a permanent calming agent.
The second caveat is dose. Teixeira and colleagues published in 2023, in Fishes, a study on meagre showing that the highest level tested degrades growth and feed utilisation, and that voluntary feed intake decreases linearly as the dose rises. The useful window is narrow.
What these studies do not demonstrate
- None involves a guppy, a neon tetra, a betta or a goldfish. The species tested are farmed or laboratory fish.
- These are factory-reformulated diets, fed over one to ten weeks. Nothing like a powder sprinkled on before a move: no immediate effect was measured.
- The measurements cover cortisol and brain activity, not observable "relaxed" behaviour, and not survival.
- No commercial product was tested in this work. It therefore validates no water conditioner sold in shops.
What remains the soundest lever
To date, what has been measured most clearly on aquarium fish remains environmental: hiding places, decor, stocking density. See our article on aquarium enrichment and chronic stress and our setup guidance in do fish feel stress. On the dietary side, basic prudence still means variety, as detailed in our guide to vitamin supplements for aquarium fish.
Frequently asked questions
Can tryptophan be added to the food at home?
That is not what these studies test: they use precisely formulated and dosed feeds, with a ceiling beyond which the effect reverses. Improvising a dose has no basis here.
Are shop-bought "anti-stress" products ineffective?
That cannot be asserted: they simply have not been evaluated in this literature. Absence of evidence is not evidence of absence — but it does rule out recommending them.
What should actually be done after transport?
Slow acclimation, darkness, no handling and no immediate feeding remain the documented steps. Find all our guides on the aquarium hub.
An accessory from our shop for this use: Programmable automatic aquarium fish feeder (€32.90) · the whole shop
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.