First the proper translation according to the google translate

waterflees=daphnia
eik=oak
beuk=beech
els= alder so it will be the props of the alder


Using the food sources named will be fine,
altough not all tads take live food.
Most live of algaes or parts of the sediment.
The tablets used for the algae eaters will sink to the bottom, same with fishfood.
It don;t get solved in the water
They just eat it and turn it to waste

Like Jace said, if the tads will have their limbs and get ashore,
they will still have their tails.
They first live of this, they "eat" their tails wich can do for up to a week.
When the tail is resolved or almost, they will start eating springtails or for larger species up to the little fruit flies.

Letting the tads check by someone with a good knowledge on them can give you an indication of the species due to certain caracteristics.
Probably they can guess the feeding habits by the mouthparts as well.

Good luck and i'm curious how they develop and turn out to be