http://www.sareefkeeping.com/SARK/showthread.php?p=8984&posted=1#post8984

and the main excerpts to date are as follows:

QUESTION: 

Would you possibly be able to post a link to your discussion on the aerating/buffering of raw RO/DI water, or if you have time maybe you could give me a quick explanation of this. I am very interested as I haven't heard of this before and am currently running top off water direct from my RO/DI unit into the sump.

Responses/points:

the gist of the demineralized water issue is rather short and sweet. RO/DI water is neutral to acidic and very unstable (crashed to low pH easily). For you to get it to saltwater useful condition... it must be buffered op to an average mineral hardness. If you do not, then adding it raw to your system is a huge burden on your ALK, CA, etc as they are reduced (ameliorated really) by the demineralized what that is coming in. A bad dilution.

Yet so much of it can be avoided. Aerating raw RO or DI water offgasses nitric acid that is lowering the RO water pH. Then adding back good minerals to buffer the demineralized water will be less of a burden on display water when they are averaged.

Using raw RO or DI water is a common cause for flow ALk or Ca in aquaria.

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Regarding freshly deminderalized water: Pending the quality of your DI resin and/or RO membrane and subsequent purity of product water, you will note that typically the raw, demineralized water is indeed very low in pH. Even when it is not acidic (rare) it is something close to neutral at best. And even in the rarest(!) cases when their is any alkaline lean to its pH... the water is categorically unstable, as defined by its lack of buffering (ALK) minerals - principally carbonate hardness.

Aerating such "weak" (buffer/ALK) water drives off carbonic acid which is temporarily depressing the pH of such demineralized product water.

So what that does is gives you a water, albeit unstable, with a higher starting pH before adding seabuffer/baking soda and/or sea salt. Thus... less of the added buffers are "neutralized" to get the raw demineralized water to the target measure of pH and/or Alkalinity you want.

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you can aerate either with an airstone or the powerhead pointing upwards, indeed.

As for aerating before kalk additions... it depends. The RO will be a burden on the alkalinity of the kalk solution, but then again... kalk slurries are so caustic that it is likely a moot point. If you are having no problems with flat pH on this effluent, then I say you don't need to aerate pre-kalk RO water.

Do note however that you CANNOT aerate kalkwasser! It will form insoluble calcium carbonate... and lots of it. Keep kalkwasser tight and as sealed as possible.

Many many thanks.

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Anthony Calfo

It has gotten me thinking though.  If your RO/DI is working correctly and you are producing 0 (or nearly 0) TDS water, the water should be very "unstable" in either direction.  It might have a low pH due to dissolved CO2, but that can be off-gassed as you stated.  However, it should also be just as unstable in the alkaline direction as well.  Adding very small amounts of alkaline substances should drive the pH up rapidly, depending on the substance of course some will drive to a higher pH than others.

If we are using RO/DI water to replace evaporated water, which theoretically should be 0 TDS, from our systems then shouldn't there be a net change of 0?  And therefore be stable as far as its effect on driving down alkalinity?

Unstable is not the correct word to use on shifts of the pH "north" (alkaline)

Deminderalized water is fairly described as unstable in that with little organic burden/matter, the pH can crash so low as to cause acidosis in fishes. Its extreme and quite miserable to see (fishes suffer badly). We are literalyl talking about a pH that can go from 7.0 to below 5.0 in mere hours/couple days! And since the pH scale is logorithymic... the 2.0 change is staggeringly shocking. Usually fatal.

But going the other way, the temporary pH (proper terminology) cannot move unassisted not will it stay high once pushed up, as influecned by atmospheric conditions and (if)any organic burden.

This is sorta the reason why FW and SW aquarists alike using liquid "pH Up" type products ride a see saw of pH up and down with doses. You are raising the temporary pH but not the total Alkalinity.

As for your assumption that only pure water evaporates... that is (mostly) true. But that does not change the fact that minerals of hardness are being extracted/reduced every day in the aquarium (a burden/draw on ALK) and that adding demineralized water is not a further burden. It's bad habit and bad husbandry really to admit raw, unstable, demineralized water to the system but then correct it after it is already in the system.

It's also a principal reason why we get so many questions about low or flat alkalinity so often

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Anthony Calfo

  I always hear that it's so important to use RO/DI for topoff, but never heard any solid advice that we should be buffering the RO/DI.  Is there a recommendation on using a specific amount of Baking soda per gallon of RO/DI or maybe add a small amount of premixed Kalk water to the RO/DI?

Thanks,

Steve

To clarify, not all RO or DI processed water is of equal or consistent quality. There are issues of membrane or resin age and quality respectively, use of prefiltration or not, and any of a number of other factors you might imagine (surge of sediments in lines from seasonal utility flushing, well water variations, etc.) that can and will influence the quality of final product water.

In short, just because water passes through an RO or DI unit does not mean it comes out pure

So you first need to test the quality of your product water. Then I suggest you buffer it up to a moderate mineral hardness. It will vary by tank and system needs (high or low mineral demand)... but in general, reconstituting RO or DIO water to a level of say 200-250ppm total hardness and/or 6-ish dKH Alk will be in the ballpark for most folks using popular synthetic sea salt brands.

As for buffering FW for evap top off... you need to decide how much if any carbonates you want to carry into the system via this remineralized water... else be sure to make it up elsewhere in the system and confirm by testing overall system water quality (mineral hardness in this case... Alk)

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Anthony Calfo

Anthony Calfo