Preparing RODI Top Off Water

It seems to us that there is a tremendous amount of mystery and confusion surrounding the preparation of R/O DI water for the reef tank.

Hopefully, this guideline will simplify the procedure and attract more hobbyists to reef keeping. If you are new to reef keeping, only RO (reverse osmosis) or DI (de-ionized) water should be used.

Top off water is seasoned fresh water (no salt added) to replace water lost by evaporation.

When the water in your reef tank evaporates, the salt does not; therefore, the salinity/specific gravity rises; fresh water must be added to replace the water lost due to evaporation and maintain the salinity and specific gravity.

In the process, you do not want to deplete your tank’s alkalinity, so a process called buffering is used. Pure unseasoned R/O DI water should never be added to a reef tank.

Seasoning RO/DI Water

To season your RO/DI water, aerate and pre-heat (77-80 degrees Fahrenheit) for at least 24 hours. Aeration drives off any carbon dioxide in the water and prevents the water from becoming stale. Water temperature should be brought up to the same temperature as the tank temperature.

After 24 hours of aeration and pre-heating, the water needs to be buffered. Buffering refers to the water’s alkalinity.

Buffer the water to between 3 and 5 meq/L or 8 to 14 dkH (multiply your meq/L by 2.8 to get a dkH value). Or target the alkalinity of your existing tank water.

Don’t be surprised if your pH goes well above the 8.1 or 8.3 values. With our water, we find the pH value goes as high as 9.00 (if a slow continuous drip system is used, the higher pH of the top off water should not alter the pH of the tank, if you notice a shift in your pH, then drip your top off water at night when the lights go out).

The primary purpose of buffering your top off water is to help maintain the tank’s alkalinity and not to deplete the alkalinity, as unbuffered water would.

Secondly, the buffered top off water should be added using a simple continuous drip system.

It is our experience that a continuous drip system for the top off water, maintains the specific gravity and helps maintain stable alkalinity (we never add buffering additives directly to our tanks), a stable alkalinity helps to maintain a stable pH.

Seachem’s Reef Builder will raise the carbonate alkalinity without immediately impacting the pH, Seachem’s Buffer can be used to raise both the pH and alkalinity.

Storing Top Off Water

Many hobbyists store their top off water in 20 gallon Rubbermaid garbage containers. Leave the water covered, aerating, and heating until you are ready to use it. To help circulate and aerate, a power head should be used rather than an air pump.