They lay the spawn on the weed, it sticks to it like glue, no good if it’s sunk, I would get some oxygenating plants in there as soon as possible. Tadpoles are meat eaters, although they do nibble the plants, you can chop up liver or tie a piece with string and drop in. I use dried fish food sometimes if there are a lot of them. You can easly tell if its dead on not fertilised, it turns white.
In that case get your pond weed in as soon as you possibly can. Get a cheap lettuce, they like that.