Bread, fries and table scraps: can you feed them to aquarium fish?
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
A crust tossed to the carp in a pond, a chip offered to the goldfish by a child: the gesture feels harmless and gets passed down through generations. Yet a review by Wilson, published in 1994 in Aquaculture, established that a fish's ability to make use of carbohydrate varies enormously between groups, and stays well below that of mammals in every case (see the study). Our feeding guides are gathered in the fish food section.
Contents · 4 sections
No fish actually needs carbohydrate
The first finding from that review is clear-cut: no dietary requirement for carbohydrate has been demonstrated in fish. These nutrients are not essential; they simply act as a cheap energy source in species that can handle them. Wilson puts the usable level of digestible carbohydrate at around 40% of the ration in a warm-water omnivore such as tilapia, but below 20% in marine, carnivorous and cold-water species such as Atlantic salmon or rainbow trout. A betta or a killifish, both firmly carnivorous, sit at the least tolerant end of that scale.
What bread and potato really deliver
Bread, pasta and chips are not just starch: they also carry non-starch polysaccharides, the fibrous fraction of plant matter. A meta-analysis by Maas, Verdegem, Wiegertjes and Schrama, published in 2020 in Reviews in Aquaculture, compiled 95 digestibility cases in tilapia and calculated an average digestibility for those polysaccharides of just 24.3% (see the study). More importantly, the authors confirm their negative effect on feed conversion ratio and on the digestibility of protein, fat and energy: this fraction is not merely poorly digested, it drags down the assimilation of the rest of the meal.
The real problem is in the water too
A piece of bread swells, breaks apart and scatters through the decor far faster than a pellet formulated to hold together. Whatever goes uneaten breaks down and loads the tank with nitrogen compounds, exactly as described in our article on overfeeding fish. A chip adds a load of fat and salt on top, which no aquarium species has any use for and which leaves a greasy film on the surface. To vary meals without taking that risk, our guide to fresh vegetables for goldfish and our comparison of flakes, pellets and wafers offer genuinely suitable alternatives.
- No dietary requirement for carbohydrate has been demonstrated in fish to date.
- 40% digestible carbohydrate tolerated in a warm-water omnivore, versus under 20% in carnivorous and cold-water species.
- 24.3% average digestibility of non-starch polysaccharides across 95 compiled cases in tilapia.
- Confirmed negative effect of that fraction on the digestibility of protein, fat and energy.
One piece of bread once a year will not kill a goldfish, but nothing justifies it: the gesture supplies no useful nutrient, degrades the assimilation of the meal and fouls the water. A food formulated for the species, topped up with suitable fresh vegetables, covers every need with none of these drawbacks.
Frequently asked questions
Can goldfish really eat bread?
They will happily swallow it, but they are cold-water omnivores: their carbohydrate tolerance stays limited, and swollen bread worsens the digestive trouble the species is already prone to.
What about a chip or a crisp, in a tiny amount?
Best avoided in all circumstances: the salt and frying fat have no equivalent in the natural diet of an aquarium fish and pollute the water straight away.
Are any table scraps acceptable?
A few raw or lightly blanched vegetables with no salt or fat, such as courgette or spinach, suit some vegetarian species — but that means plain vegetables, not leftovers from a cooked meal.
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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.