Reverse Osmosis water being Acidic


New member
I posted this in the reef chemistry section also, but thought I might be more responses over here.
I know everyone is now going to reverse osmosis water for the reef tank. But during the RO process, it produces acidic water. Does the acidic level not harm the live stock in the reef tank?Wouldn't long term use of RO eventually cause harm to the system, espically when you are doing 10-15% water changes weekly? Wouldn't this cause the essential minerals that we are adding to the tank (calcium) to be stripped from the corals in order to neutralize the water? Does anyone have any thoughts on this?


Premium Member
I always thought RO/DI water was neutral and not acidic but never gave much thought to it. As long as TDS is 0 ~ thats all I worry about.

A high quality salt mix will replace any beneficial minerals that are lost and bring the water to the correct PH levels.

When topping off you are only replacing water that has evaporated and all the minerals are left behind. Top off water can bring down your PH depending on the size of tank and amount added. Some people add kalk to the water and drip it at night to keep PH up and stable. I add my top-off water during the day when PH is at its highest.
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In Memoriam
Measuring the pH of RO/DI water is meaningless, there is nothing to measure. You will get false readings that may register high or low but it really does not matter.

old salty

Premium Member
The purpose of using RO/DI water is to initially strip all the contaminants out of the tap water. When you add salt, you are re-contaminating the water in a controlled way. The essential elements are contained in the salt in quasi-controlled portions. If you include the elements in tap water, it may throw the portions out of balance. Adding RO/DI water for topoffs ensures that you are not adding additional contaminants to the water.

Measuring the pH of RO/DI water requires a very specific pH probe and they are very expensive.


Premium Member
The water does absorb CO2 giving it an acidic pH, but it can't be accurately measured to begin with. If it never contacts the atmosphere, it would be pure with a theoretical pH of 7.0.