RO-reverse osmosis This kind of water has been stripped off its nutrients. Your fish must have suffered osmotic shock. How did you acclimate them?fishbabe;2373137; said:what is R.O water, we just got the water from the machine that sold the water in large bottles. the tank is perfect my dad brought 5 books from the store, and set the tank for me. we put the discus with the full grown one that i had for 3 months.