Thiamine (vitamin B1) is essential for carbohydrate metabolism and nervous system function in freshwater fish. It serves as a coenzyme in the metabolic pathways that convert food into usable energy, particularly the breakdown of carbohydrates and some amino acids. Without thiamine, the nervous system cannot function properly, leading to neurological symptoms that are among the most dramatic of any vitamin deficiency in fish.
Thiamine deficiency has been well-documented in fisheries science, where it is associated with a condition called Early Mortality Syndrome in wild fish populations. In aquarium settings, thiamine deficiency is most commonly caused by feeding raw fish or shellfish that contain thiaminase — an enzyme that destroys thiamine. Certain feeder fish, raw mussels, and raw shrimp contain thiaminase, and fish fed heavily on these items can develop deficiency even though their overall diet seems adequate. Cooking or freezing destroys thiaminase, making frozen foods much safer than raw in this regard.
For aquarium hobbyists, the practical takeaway is to avoid feeding raw freshwater fish or raw shellfish as a staple diet. Frozen foods are safe because freezing and the subsequent thaw cycle degrades most thiaminase activity. Commercial flake and pellet foods are supplemented with thiamine and are not a risk factor.
Quality commercial fish foods are supplemented with thiamine and provide adequate levels. Avoid feeding raw freshwater fish or raw shellfish as a significant portion of the diet, as these may contain thiaminase that destroys thiamine. Frozen bloodworms, brine shrimp, and daphnia are safe. If you breed live feeder fish, gut-load them with quality flake food before feeding them to your predators.
0.09% of daily nutrient intake
Vitamin B1 (Thiamine) makes up 0.09% of your freshwater fish's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Neurological symptoms including loss of equilibrium, spiraling or erratic swimming, head tremors, convulsions, and loss of the righting reflex (fish unable to stay upright). Also loss of appetite, darkened coloration, and in severe cases, sudden death. Deficiency develops relatively quickly — within weeks — in fish fed thiaminase-containing raw foods as a primary diet.
Thiamine is water-soluble and excess is readily excreted. Toxicity from dietary sources is essentially unknown in fish. There is no practical risk of thiamine excess from normal feeding.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 1 | 10 | mg/kg diet | Higher requirements when feeding thiaminase-containing raw foods (certain feeder fish, raw shellfish). Freezing largely inactivates thiaminase. |
Source: NRC 2011, general aquaculture consensus
