At Okayama University of Science (OUS), the groundbreaking Third Water™ makes it possible for marine and freshwater fish to coexist in the same tank. This innovative water is featured in a 2-ton aquarium on the first floor of the 50th Anniversary Commemorative Building (1-1 Ridai-cho, Kita-ku, Okayama City), owned by Kake Educational Institution, the parent organization of OUS . Visitors are welcome to view this unique display on weekdays from 9:00 AM to 5:00 PM. OUS reintroduced this fascinating exhibit following the addition of more clownfish and goldfish.
The aquarium currently houses 15 clownfish, 4 electric blue damselfish, 5 Ryukin goldfish, and 5 Mitsuo Wakin goldfish. The clownfish seem particularly fascinated by the Ryukin goldfish’s flowing tails and are sometimes seen chasing them. Territorial behavior and subtle tension between the species creates a dynamic interaction that keeps observers engaged. Meanwhile, the damselfish swim gracefully through the tank, seemingly unaffected by commotion around them. While this mix of species would never occur naturally, their interactions are fascinating to watch and offer an entertaining glimpse into their behaviors.

What is Third Water™?
Third Water™ is a specially formulated water developed by OUS. By isolating key components like sodium, potassium, and calcium needs for marine fish, this water achieves a salinity level of just 0.5-1.0 %, far lower than typical seawater. This low salinity allows marine and freshwater species to coexist in the same environment. 
OUS’s Education and Research Center for Organisms Production at the Okayama campus is home to large-scale tanks, including four 35-ton tanks and one 140-ton tank. Over 20 species, such as tiger pufferfish, Japanese eel, bluefin tuna, sockeye salmon, grouper, flounder, and Pacific white shrimp, have been successfully farmed and shipped from this facility. Using Third Water™, rare and high-value species like the giant grouper are cultivated in Tsuno, Miyazaki Prefecture, while inland aquaculture of sockeye is conducted in Fukushima City.