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Old 02-25-2008, 10:17 PM   #1
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RO water Ph

Why does my RO water Ph keep changing when making top off water. I am using a 32 gallon trash can with an air stone. I am also using Reef Buffer to set the Ph. I will treat the water until I get a Ph of 8.4 but two days later it will be quite a bit lower than that. I am waiting at least a half day before testing after adding the Reef Buffer. What is the secret to getting the wanted Ph - say 8.4? There must be a trick to it that I have missed. Thanks for the help.
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Old 02-26-2008, 12:42 AM   #2
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Well, O2 exchange (in SW) is at the surface of the water. An air stone doesn't really help cause much of that (the bubbles are very small, so they don't cause much ripple or surface agitation). Try aiming a ph at the surface to cause more O2 exchange at the surface.
Also, do you have a cover on the tank? That can inhibit O2 exchange.
PH will also swing (especially in new tanks) make sure you take the readings at the same time each day.
PH will be lower after lights out and higher after the lights come on.
Can you give us any more info about your tank?
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Old 02-26-2008, 02:03 AM   #3
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I think liontech is saying he's trying to get his RO water to reach 8.4... not his tank water.

Liontech... if that's the case, you don't need to buffer your top off water. I think most folks just use it straight from the storage bin. Works fine as long as you're not adding a ton of top off water at once... which you shouldn't be to start with!
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Old 02-26-2008, 09:19 AM   #4
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I think liontech is saying he's trying to get his RO water to reach 8.4... not his tank water.

Liontech... if that's the case, you don't need to buffer your top off water. I think most folks just use it straight from the storage bin. Works fine as long as you're not adding a ton of top off water at once... which you shouldn't be to start with!

CO2 maybe? Carbonic acid of course will drop the pH, do you get enough introduced with the stone to make a difference?
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Old 02-26-2008, 11:56 AM   #5
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I'm no chemist, but I think the reason is that because RO water is basically stripped of any ions that give water "pH", any little change in those ions will cause drastic swings in pH. That, in addition to the fact that RO water has no alkalinity reserves built in (alk = 0) means that there is no buffer to those swings. Granted... you are adding "buffer", but you'd need to add quite a bit to get it to levels that would stay stable. That's a lot of what your salt mix does.

Here's a good article, with a link to the direct spot that talks about pH of RO water...

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria by Randy Holmes-Farley - Reefkeeping.com

From that article, specifically, "Many aquarists with low pH problems have asked, for example, if their aquarium’s low pH may be caused by their replacing evaporated water with RO/DI water that they measure to have a pH below 7. In short, the answer is no, this is not a cause of low pH nor is it something to be generally concerned about..."
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Old 02-26-2008, 09:51 PM   #6
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Kurt, Thanks for the article. I do also use the RO water for making tank water for water changes. But, I have read where the RO only needs to be close and the salt mixture will take care of the Ph. The problem was that I could never get it to stay close. The question now is how does one maintain a 8.2 in the tank. This has bothered me for quite a while and I always felt I was doing something wrong.
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Old 02-27-2008, 02:19 AM   #7
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Now we get back to roka64's comments earlier in this thread! See his comments about maintaining pH in your main.

Basically... tightly covered tanks don't help maintain high pH. Surface ripple will help increase pH. Understanding where your alkalinity is at will help you troubleshoot also. What are your exact pH levels at the end of the day just as your lights go out? And what are your alkalinity levels? And while we're at it, what salt mix are you using and how often are you doing water changes and how much?

Low pH can be from a lot of different sources. Here's a good article that might give you some ideas of where to start...

Chemistry and the Aquarium
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Old 02-27-2008, 09:41 PM   #8
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Kurt, I briefly looked at the "chemistry and the aquarium" info you gave me. Good stuff. I am preparing to go to Vegas for a week so, when I get back, I will study it and start recording the test readings as well. I will then be able to provide you with better information.
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Old 03-01-2008, 12:17 PM   #9
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If anyone is interested, and has read the article, as to what 'pH' means (the actual term):

The lowercase 'p' is actually a mathematical expression and not a 'letter'. The 'p' means 'negative common log'. The 'H' refers to the H+ (hydrogen ion) concentration. Concentrations are expressed in what is known as molarity, the # of moles/liter. A mole is a large # of particles, 6 x 10^23.

At a H+ concentration of say 10^-8 M (0.00000001 molarity) the pH would be 8. As the H+ concentration gets weaker (10 ^-9, 10^-10 etc.) you can see the exponent (-8, -9 ,-10) is getting smaller, the negative of this (8, 9 ,10) gets larger. That is why was acid levels decrease the pH increases.

Some texts refer to the term 'pH' as meaning the 'potential of hydrogen' or 'percent of hydrogen'. This is absolutely incorrect.

Water has a property in that the product of the H+ ions and the OH- ions is always 10^-14. That means that the pH and pOH (yes, pOH is real) always add to 14. If you were tracking alkalinity and measuring pOH it would get larger as alkalinity decreased. At a pH of 8 the pOH is 6.

The 'p' can be used with other measurements as well: pKa (acid dissociation constant), pM (metal ions), pOH, whatever.
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Old 03-01-2008, 05:26 PM   #10
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I maintain 8.2 with weekly PWC`s using oceanic marine salt. What brand are you using?
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