If you want to rule out dropsy, test the water with a kit. There are strip tests that are relatively inexpensive, but they are generally not as accurate as the kits that are about 50$. That will tell you a lot about the water they are in. It could be because of water quality, but since it's a couple and not a lot, who knows.

If the PH is too high, tannins may help. These can come naturally from certain leaves or drift wood.

We put a leaf in our tadpole cups, and change them regularly as the leaves do run out of it basically.

Anyway, that's my 2cents. Hope it helps