My sincere apologies, I should have updated the thread. He sent me a pm with all the details.

The rocks were rocks that he bought at a store, he first boiled them, and then froze them before adding them to the tank.

The tadpoles have hind legs, but no front legs, and they seem to be breathing atmospheric air. I suggested that he bring down the water level, because it was 14 cm deep, and the tadpoles naturally live in the shallow areas of marshes.

He does not filter the water, but has aquatic plants, and does weekly water changes.

Apart from the change in underside color, the tadpoles are acting completely normal. So, I didn't think that it was currently anything to worry about, and told him to let us know if there are any changes. And, if there are any other problems, suggested moving them to a clean tank, without the risk of contaminating substances, and then work from there.

I don't know if my advice was any good, but I mostly raise tadpoles, and only asked questions about things that I have found problematic in my experience.

Sincerely,
Joh