Topping off with RO water

[Jeff_F]

I use RO water because our well water is very bad. I remineralize  with APT Sky and small amount of alkaline buffer.  My GH is 6, KH is 3 and PH goes between 7.1 with CO2 off and 6.1 when on.

The PH of my RO water is around 5.5.  My aquarium has a glass cover over about 3/4 of the top so I do get evaporation.  So….should I top off with just raw RO water?  It is a 20 gallon tank and I top off with about 2 quarts every couple of days.  If not, how do I maintain the ph without affecting other parameters.

[Tommy Vercetti]

You have it correct. the PH of the RO water is very low because it has no minerals to buffer it. it will will correct to the proper PH, when added to your aquarium for top off purposes, because when that water evaporated it left behind the minerals.

[tolstoy21]

On 8/2/2023 at 12:44 PM, Tommy Vercetti said:

You have it correct. the PH of the RO water is very low because it has no minerals to buffer it. it will will correct to the proper PH, when added to your aquarium for top off purposes, because when that water evaporated it left behind the minerals.

Technically, RO has no Ph, or is Ph neutral (7.0) until acted upon by something that either acidifies or alkalizes it.  It's kind of like a blank slate waiting to be influenced. More often than not, in situations where aquarists use it, it gets acidified by wastes in the tank, unless the tank is full of crushed coral or aragonite, or some other non-inert rock.

If you make RO in a bucket and let it sit a day, its Ph should read 7.0 after it gasses off any CO2 that might be dissolved in it. You are correct, RO water has no buffers, which is why it's completely neutral in its pure state.

On 8/2/2023 at 12:40 PM, Jeff_F said:

So….should I top off with just raw RO water?  It is a 20 gallon tank and I top off with about 2 quarts every couple of days.  If not, how do I maintain the ph without affecting other parameters.

Yup. That's exactly what you should do, and what many people do, especially those keeping reef tanks. This way, the parameters of the water stay the same because you're adding pure water, and not something with mineral content that would add to the already present mineral content of your aquarium. In a reef tank, RO top offs keep the salinity from increasing. Whatever is already in your tank will influence the Ph of the RO water either up or down so that it matches the existing water params. However, if you displace your tank water with RO water (meaning do a water change), you would be diluting the buffers, so this would drive the Ph down.

Edited August 3 by tolstoy21

[Tommy Vercetti]

Thanks, but if you after the real cheese, then:

 

The pH of pure water (H20) is 7 at 25 °C, but when exposed to the carbon dioxide in the atmosphere this equilibrium results in a pH of approximately 5.2 because CO2 in the air dissolves in the water and forms carbonic acid.

[tolstoy21]

On 8/2/2023 at 9:42 PM, Tommy Vercetti said:

Thanks, but if you after the real cheese, then:

 

The pH of pure water (H20) is 7 at 25 °C, but when exposed to the carbon dioxide in the atmosphere this equilibrium results in a pH of approximately 5.2 because CO2 in the air dissolves in the water and forms carbonic acid.

Learn something new every day!  Thanks.