Hyalinobatrachium valerioi.
Thanks to @Gigit for posting her photos of these fascinating little frogs and alerting us that they are on show (but unlabelled). The bottom 2 images show a female, I think, and the others show a male. In the morning they sat on the wet front glass of their tank, showing their transparent bellies, images on the left. In the afternoon, when the glass had dried, they sat under the leaves, shown on the right. The central image shows a frog on the glass side of the tank. Zootierliste says that these frogs arrived from Slimbridge,
