# Adding RO Water to reduce PH



## Kai McNeil (27 Oct 2016)

Hi All,

I'm trying to lower my PH, its currently always around 7.9 - 8.1 from the tap.

I have easy access to RO water, if i add a certain percentage of RO water during a water change will this help me reduce the PH levels?

I should also add, that i have a heavily planted tank, running C02. 

Thanks All,


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## Dr Mike Oxgreen (28 Oct 2016)

What is your reason for wanting to lower pH? Do you have particular fish that require it? And what pH are you aiming for?

If you do decide to alter pH, you'll need to know what the KH hardness of your water is. KH is a measure of how much carbonate there is in the water, and this is what profoundly affects the pH equilibrium. If your tapwater KH is high (which it probably is), you will not succeed in lowering the pH without lowering the KH.

You can indeed lower the KH by mixing tap water with RO or rain water; in fact this is probably the best way of doing it. The RO water will be zero KH, so mixing 50:50 will halve the KH of your tapwater - and likewise you can reduce the KH by other proportions by choosing the ratio of RO and tap water accordingly. My tap water is pretty hard with a KH around 15°, so I mix it with rainwater (25% tap, 75% rain) to give a KH of 4°.

If, for example, you're aiming for a pH of 6.5, you'll probably want a KH of somewhere around 4-6°. If you establish that KH then your CO₂ will do the rest. Don't go much below about 2°-3° though, because the pH could become unstable below that level and could crash, becoming excessively acidic.

Mixing RO and tap water will also lower the GH hardness, which is a measure of Calcium and Magnesium.

Bear in mind that any changes to water chemistry must be done slowly, so use small water changes to start with.


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## dw1305 (28 Oct 2016)

Hi all, 
Mike is right. The problem is that pH isn't a very useful measurement without some estimation of carbonate hardness (dKH). 

The pH8 you have in your tap water is the pH that carbonate buffered water has due to the <"carbonate ~ CO2 equilibrium"> at ~400pppm atmospheric CO2. 

When you just add RO water, you will reduce conductivity, dGH and dKH, but the *pH won't change,* until it becomes unstable as you approach pure H2O. 

If you have a source of acids (like H2CO3 from the dissolution of CO2) the amount that they cause the pH to  drop will depend upon the carbonate buffering in the water. 

Because of this pH isn't a very useful measurement in very soft water, small changes in water chemistry (the ratio of acids and bases) <"cause large fluctuations in pH">. This happens naturally in vegetated soft water as photosynthesis alters the ratio of CO2 (an acid) and dissolved oxygen (a base).  

This is from THE <"Ecology of the Planted Aquarium">





> "_Daily variations of the water parameters are rarely – if ever – taken into account. Data collected in a freshwater lake (Star Lake, VT) with a very low alkalinity showed a diurnal pH fluctuation beyond the imagination of most hobbyists. Thus, the pH at 10 am was measured at 5.7 (strongly acidic), 9.6 at noon (strongly alkaline), 8.3 at 2 pm (moderately alkaline) and finally, 6.4 at 4 pm (slightly / moderately acidic). Readings were taken at a 0.5M depth. The fluctuation observed was due to the low KH value of the water (something reported for the Amazon river, too) and the presence of large amounts of phytoplankton. Under the circumstance it is extremely difficult, if not impossible, to figure what is the “right” pH for any form of aquatic life collected in that lake and which tank could cope with this kind of fluctuation. The low – high points of the day differ by 4 pH points, which means that the concentration of H+ in the morning is 10.000 times higher than at noon, while this change takes place in just two hours. It goes without saying that this pH swing cannot be observed in an aquarium only because we cannot reproduce the amount of light which falls in the Lake. In any case, if somebody reported that a suitable pH for aquatic life collected in this lake should range daily from 5.7 – 9.6 most hobbyists would think it was a typo._"


Cheers Darrel


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## Kai McNeil (1 Nov 2016)

Thanks for the replys all. This is where i'm confused, The Kh/Gh testing is this linked with conductivity? i have a fluval G3 filter and my conductivity level is always high at 870.


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## dw1305 (1 Nov 2016)

Hi all,





Kai McNeil said:


> my conductivity level is always high at 870


The conductivity  (in microS) is a measure of all the ions in solution, but it doesn't tell you which ones they are. 

It is a high conductivity reading for fresh water, and it probably isn't all dGH/dKH from CaCO3, there will be some other ions (Na+ probably). 

Conductivity and pH aren't necessarily linked (acids (H+ ions) and neutral salts like NaCl will raise conductivity), but usually, in the UK, high conductivity water will be strongly carbonate buffered, making reducing the pH difficult.

cheers Darrel


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