What you need to know about predator fish and thiaminase


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We now have a better understanding of appropriate diet for predator fish and the role of enzymes in keeping them healthy. Neale Monks has the details.

In the past aquarists often threw in a few feeder fish, assuming a live diet was the right food for the flesh eaters. Yet all too often predatory fish kept in that manner exhibited poor health and died prematurely.

Parasites were one reason live feeder fish caused problems — bacterial infections another. However, we have become increasingly aware of an enzyme called thiaminase often found in feeder fish species most widely used as food — such as goldfish and minnows.


Enzymes are chemicals found in living things that work to speed up chemical reactions.

Thiaminase breaks down Vitamin B1, also known as thiamin. In the right place it’s useful but, from the fishkeeper’s perspective, a menace!

Vitamin B1 is as important to our fish as it is to ourselves. Without adequate supplies, humans develop a sickness called beriberi — and symptoms include lethargy, weight loss, problems with nerve control and sense organs, heart failure, and eventually death. Fish will develop similar symptoms.

Because it’s a water-soluble vitamin, B1 is easily lost to the environment and cannot be stored in the body, but a daily input is essential to good health.

Fat-soluble vitamins can be stored, so a daily supply isn’t important provided the animal can occasionally top up its fat-soluble vitamin stores.

This isn’t a problem if you feed your fish flake or pellet foods, because these will have carefully measured quantities of all the vitamins needed. However, if using live, fresh or frozen foods without supplementing with flake and pellet foods, you have to be far more careful.

The live, fresh or frozen foods used must be biased towards species that do not have thiaminase. Occasionally using foods with it isn’t a problem, but if thiaminase-rich foods are used most or all of the time, there’s a very good chance of a health risk.

There are really only three groups of foods dangerously rich in thiaminase: mussels, crustaceans, and cyprinid fishes. Mussels are popular fresh or frozen foods because they’re cheap and readily available. Crustaceans are many and varied, but we’re concerned with prawns and shrimps widely used as fresh or frozen foods.

Safe foods

The cyprinid family contains goldfish, carp, minnows, danios and their relatives. Although not legally used as feeder fish in the UK nowadays, the habit continues elsewhere in the world.

What foods don’t contain thiaminase? Cockles, tilapia, coley, cod, and haddock are thiaminase free — as are the smelt sold as lancefish.

Terrestrial foods generally don’t contain thiaminase either, so you’re safe with earthworms, bloodworms and crickets.

Does cooking destroy thiaminase and make thiaminase-containing foods like shrimps safe? Cooking will destroy thiaminase, but you can’t know how much Vitamin B1 was lost before that item was cooked. So such foods may contain some Vitamin B1, but they should still be used in relative moderation — if at all.

Are gut-loaded shrimp safe? Probably, provided the shrimps were fed a quality flake food beforehand and you can see their digestive tracts filled with nutritious flake. While an expensive way to feed long term, this is good way to get newly imported predators feeding and settled.

If all else fails, use vitamin supplements to boost the nutritional value of live, fresh and frozen foods.

However, it’s often more convenient to wean the predators on to at least some flake or pellet foods alongside more meaty fare. Do that and the problem should pretty much disappear.

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