The black dragonfish (Idiacanthus atlanticus) is one of the weirdest looking of the dark sea hunting fish that we know of. It’s likely many more exist that we don’t know about that are just as if not more weird. It almost exclusively hunts in the dark of the ocean and it’s bioluminescent light is almost invisible to human eyes, it however is able to see and use it to hunt its prey.
Thulium is a fairly rare “earth” element, it maintains this name despite being available in sufficiently detectable quantities in the ocean. It’s mostly used in lasers and X-ray machines. All this makes it a suitable candidate for being tied to the black dragonfish.
Ytterbium (not to be confused with Yttrium, it’s one of four elements named after the same Swedish Town) is the second last lanthanide. It’s mostly used to help form stainless steel, in lasers, and as a source of gamma rays. It will react fairly readily with oxygen outside of an alloy and forms a white salt.
Tench are a fresh water fish, though “fresh” is stretching it as they tolerate very low oxygen environments that even Carp has trouble surviving at. This is why it’s paired with Ytterbium.