High ph of RO water

[ludlumjohn]

Hello,

New to this site but have been keeping fish and plants for more than 50 years, yes I’m old 🤪

Currently running 2 planted tanks with C02 and doing PPS auto dosing. Tanks are only about a month old. The startup went very well, absolutely no algae issues other than some minor diatom issues after the 1st couple weeks.

I have available both RO and well water available and I trying to get the right mix of both to end up with about a 7.5 ph.

I set aside two glasses of water from the well and RO and had the following readings straight from the tap.

RO 6.2 ph and a tds of 10
Well 7.07 ph and tds of 220

After airating for a few hours I end up with the following
RO 7.95
Well 8.64
Any ideas on why my RO water would go so high. I want to reach equilibrium but curious if the airation is throwing it off and I should just let it settle.

The 2 tanks that are running now I did a 75 RO and 25 well. Tanks read about 7.0 in the morning and down to about 6.1 after c02. I would prefer to start with 7.5 and go down to 6.5 after c02. When I set these two tanks up I just did my normal mix of the 75/25 but was surprised how low is is. This is after a water change (2-3 days. I was thinking I would end up with a 50/50 mix but confused by my RO reading and my tanks actual reading. Hoping someone can make some sense of this for me.

Thanks in advance

 

[dw1305]

Hi all,

Any ideas on why my RO water would go so high

After airating for a few hours I end up with the following
RO 7.95
Well 8.64

The problem is <"with the lack of solutes"> in the RO <"water">, it just means that pH is a moveable feast.

I want to reach equilibrium but curious if the airation is throwing it off and I should just let it settle.

It shouldn't be, you could only get a higher pH value if you weren't at equilibrium with atmospheric CO2 levels, e.g. CO2 levels were below the equilibrium point.

The 2 tanks that are running now I did a 75 RO and 25 well.

That sounds about right as a mix to me, you should have about 60 ppm TDS? ((0.75 x 10) + (0.25 x 220) = 63 ish)

would prefer to start with 7.5 and go down to 6.5 after c02.

That is trickier, because we don't know which ions are in the well water and pH isn't a linear scale like conductivity (ppm TDS). Why do you want to start at pH 7.5?

I was thinking I would end up with a 50/50 mix

That should also be OK. Are you near the sea? If you are you might have sodium (Na+) ions in the well water, which wouldn't be great.

If you are away from the sea? And particularly if the geography is limestone (CaCO3)? The ions will be overwhelmingly calcium (Ca++) and bicarbonate (2HCO3-).

cheers Darrel

 

[ludlumjohn]

Thanks for the awesome reply.

One at a time per your reply.
I’ve heard measuring ph of ro water can be trickle because it devoid of almost everything. Should I assume my RO water is the ph at which it comes from the tap? It also sits in holding tanks before I tap it and that’s has to be a day or 2.

As for the TDS based on what I start with that should be about right but Bcz of dosing and bio load I’m about 125.

As for why 7.5, I guess having lived with high ph is previous locations guess I’m just nervous with a ph in the low 6’s. I guess also if I could use a little less RO that would also be good, so a 50/50 mix would help there. I live in northern Thailand. No ocean in sight.

Curious if you have done any pps dosing. It recommends in lieu of weekly w/c to do a tds test. When your TDS in the tank is greater than 300 plus original water in use for the change then do a change. Any thoughts of experience with that?
Thanks again!
My god I love this hobby

 

[dw1305]

Hi all,

I’ve heard measuring ph of ro water can be trickle because it devoid of almost everything.

Yes that is it, pH isn't a useful measurement in RO because of the lack of solutes.

Should I assume my RO water is the ph at which it comes from the tap?

You can just ignore the pH of the RO, it is a blank slate.

As for why 7.5, I guess having lived with high ph is previous locations guess I’m just nervous with a ph in the low 6’s.

No need to be, the pH reduction due to CO2 addition doesn't reduce the alkalinity, you just have more Total Inorganic Carbon (TIC) <"dissolved in the water">. That is one of the problems with pH, you <"can't really extrapolate"> from hard to soft water.

I guess also if I could use a little less RO that would also be good, so a 50/50 mix would help there. I live in northern Thailand. No ocean in sight.

I'd go for that. I use rainwater in my tanks, but if I didn't have rainwater? I would use our (very hard) tap water.

Curious if you have done any pps dosing. It recommends in lieu of weekly w/c to do a tds test. When your TDS in the tank is greater than 300 plus original water in use for the change then do a change. Any thoughts of experience with that?

I change a small volume of water most days, but I use a combination of the <"Duckweed Index"> and <"tank conductivity"> for <"tank management">.

cheers Darrel

 

[Andy Pierce]

I asked ChatGPT to make a table that would show "the calculated equilibrium pH values for water at 20°C with varying alkalinity (measured in dKH) and CO2 levels. The CO2 levels are presented as atmospheric concentration (ppm) and the resulting dissolved concentration in water (ppm)." and below is what it came up with.

To @ludlumjohn original request, for a low tech tank and if the numbers are to believed, with what might be typical indoor CO2 at 600 ppm in air, you'd need a dKH of 6 to get a pH of 7.5.
For CO2 injected tanks, having a final water CO2 at 30 ppm is equivalent to not injecting CO2 if there were 20000 ppm CO2 in atmosphere.

Down: Alkalinity (dKH) Across: CO2 (atm ppm / water ppm)	400 / 0.6	500 / 0.8	600 / 0.9	700 / 1.1	800 / 1.2	900 / 1.4	1000 / 1.5	2000 / 3.0	20000 / 30.3
1	6.7	6.6	6.5	6.4	6.3	6.2	6.1	5.8	4.7
2	7.0	6.9	6.8	6.7	6.6	6.5	6.4	6.1	5.0
3	7.2	7.1	7.0	6.9	6.8	6.7	6.6	6.3	5.2
4	7.4	7.3	7.2	7.1	7.0	6.9	6.8	6.5	5.4
5	7.6	7.5	7.4	7.3	7.2	7.1	7.0	6.7	5.6
6	7.7	7.6	7.5	7.4	7.3	7.2	7.1	6.8	5.7
7	7.8	7.7	7.6	7.5	7.4	7.3	7.2	6.9	5.8
8	7.9	7.8	7.7	7.6	7.5	7.4	7.3	7.0	5.9
9	8.0	7.9	7.8	7.7	7.6	7.5	7.4	7.1	6.0
10	8.1	8.0	7.9	7.8	7.7	7.6	7.5	7.2	6.1
11	8.2	8.1	8.0	7.9	7.8	7.7	7.6	7.3	6.2
12	8.3	8.2	8.1	8.0	7.9	7.8	7.7	7.4	6.3
13	8.4	8.3	8.2	8.1	8.0	7.9	7.8	7.5	6.4
14	8.5	8.4	8.3	8.2	8.1	8.0	7.9	7.6	6.5
15	8.6	8.5	8.4	8.3	8.2	8.1	8.0	7.7	6.6
16	8.7	8.6	8.5	8.4	8.3	8.2	8.1	7.8	6.7
17	8.8	8.7	8.6	8.5	8.4	8.3	8.2	7.9	6.8
18	8.9	8.8	8.7	8.6	8.5	8.4	8.3	8.0	6.9