The direct answer for a betta
Food made for other kinds of pets is not safe as betta food, and a betta should eat its own species-appropriate diet. A betta's natural menu is insects and small invertebrates taken at the surface, and its body is built for concentrated protein in very small portions. Dog kibble, cat food, and foods blended for other animals are formulated for completely different digestive systems, energy needs, and mouth sizes. A crumb a betta snatches once is unlikely to be a crisis, but making any of those foods a habit, or a substitute when the betta pellets run out, is where the harm builds.
Why cat and dog food fail a betta
Cat and dog foods fail a betta on size, richness, and balance all at once. A single piece is huge relative to a betta's stomach, softens into a greasy lump in warm tank water, and fouls a small aquarium faster than the filter can cope. Their fat levels and plant fillers suit a mammal that chews and digests over hours, not a betta whose short gut is designed for small, frequent insect meals. Wet cat food is no better in the tank: it clouds the water within minutes and a betta can gorge on it before you can net the excess out. Keep other pets' bowls away from the betta tank and their bags sealed where loose pieces cannot be dropped in by a curious child.
Why other fish foods are only a partial match
Food for other fish is closer, but still a poor staple for a betta. Goldfish food is deliberately plant-heavy and lower in protein, so a betta kept on it tends to lose color and condition over time. Generic tropical flakes vary widely, and many are built around plant meals that a betta picks through, eating the richest flakes and leaving the rest to decay. A plain tropical staple for a day or two while proper betta food is on its way is a reasonable emergency bridge, nothing more. The betta should return to a pellet or food made for carnivorous surface feeders as soon as possible, in the small portions its stomach expects.
What to do if a betta has eaten the wrong food
If a betta has eaten a little food meant for another pet, remove whatever remains in the tank, check the water is clear, and simply return to its normal food at the next meal. Watch the betta over the following day for a swollen belly, sluggishness, or food being spat out repeatedly, and skip a meal if it looks uncomfortably full rather than adding more. Test or change water sooner than planned if a rich mammal food has melted into the tank, because the pollution harms the betta more than the mouthful did. If a betta that ate the wrong food stops eating, cannot stay level, or deteriorates, contact a veterinarian familiar with fish rather than trying a sequence of home remedies.
Keeping the betta's menu separate at home
In a multi-pet household the mix-up usually happens at feeding time, when one person feeds every animal in a rush. Store the betta food beside its tank and the other pets' food in its own place, and make the rule explicit that nothing from a dog or cat bag ever goes in the water. Children in particular love feeding the betta and need the boundary stated as part of the treat. With KinsLe Care Teams, the betta feeding duty can carry that own-food-only rule in writing, so every helper feeds the fish its proper pellets and nothing borrowed.