I feed these tadpoles with flaked goldfish food. They hatch a lot faster than the tadpoles left in the pond. I keep them in the house until the tadpoles in the pond hatch. I then put the indoor tadpoles back into the pond and continue to feed them goldfish flakes each day. The others in the pond soon learn that it is food and you soon have a black squirming mass of tadpoles around the edge of the pond whenever you feed them.
Several pinches disappear in seconds.
Do not be tempted to add more - wait until the next day. I discovered some of last year's tadpoles in one of my ponds. They are medium size tadpoles, but with no legs evident yet. I have never come across this phenonemon in my childhood. The pond in question is just a bowl in the ground in the garden and is in a 'normal' position in terms of light etc.