Im Aquarium der Wilhelma

Dieser Fisch hat mit dem Mund etwas Bodengrund aufgenommen und dann kleine Kiesel durch die Kiemen wieder ausgespuckt... (So sah es jedenfalls aus.)

This fish "ate" some ground matieral, and then spat out little stones through his gills... (At least that's what it looked like.)
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Three species share the aquarium. The only one on the sign that resembles this fish is Lythrypnus dalli, but - judged by the photos on Wikipedia - that fish should be red.

Can you help?