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Is there a point that pea gravel will stop raising my ph ?  Or should I give up and throw it out and start over.  (I did the vinegar test and it bubbles like soda, but it was the only source available here.) I was hoping that it would give up after a while...  

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Yes, there is a point where your "pea" gravel will stop raising your pH. And that would be when the calcium carbonate content is (almost) all used up...The good news is bacterial respiration is an acidifying process...

The further "good" news is, if you look at it from a "geological time" perspective, it wouldn't take all that long...In human AP time however, it could be looked at as never (depending on the exact mineral make-up of your gravel, this could be deemed either an 'uniquivical' never-ever-never, or as an 'almost' never...as in - ...your great-great grand kids might live to see the day...- type of never) 

Hope that this helps you choose a suitable course of action to achieve your system pH goals...



Vlad Jovanovic said:

Yes, there is a point where your "pea" gravel will stop raising your pH. And that would be when the calcium carbonate content is (almost) all used up...The good news is bacterial respiration is an acidifying process...

The further "good" news is, if you look at it from a "geological time" perspective, it wouldn't take all that long...In human AP time however, it could be looked at as never (depending on the exact mineral make-up of your gravel, this could be deemed either an 'uniquivical' never-ever-never, or as an 'almost' never...as in - ...your great-great grand kids might live to see the day...- type of never) 

Hope that this helps you choose a course of action...


Well also on the good side, I have an automatic carbon dioxide machine for my plants.  (just add white vinegar) 

Yeah, you can add just about any (non anti-bacterial) acid to help that process along...HCl being the most fish beneficial and effective IMO...They may not appreciate the constant pH 'bouncing' though...and if your gravel is mostly limestone, by the time you do dissolve all the CaCO3 you might not be left with much media...

In a 55 gallon system it might be best just to replace the media.

where are you located?  I might be able to help you with the media.

There are ways of estimating how long it'll take, but the likely answer is more than the life of your system. Just replace your media--or something more creative, like build a raft bed.

OK,  I changed out the media in my grow beds to expanded shale  (PH neutral right?) My feed water is at 7.6 PH and every night it goes up to about 8.4.  What is going on?  I have heard that the dust in the expanded shale may have to do with this.  [I rinsed the shale out before using it] This is the only problem I am having, as the amonia and the nitrites and nitrates are all acting like they should...  Any suggestions?  

Doug. Take a glass. Fill it with your "feed water" (I'm assuming you mean tap or source water). Let it sit out in that glass for a couple of days THEN test the pH. I'm betting that the pH is going to be higher than 7.6. (which is probably the reading you are getting when you test it straight otta the tap). Do yourself a favor, and test your source water after it's sat out a couple of days.

Your water may in all likelihood be closer to 8.2-8.3 if you are testing it right out of the tap without letting it sit out first for a few days to off-gas the carbon dioxide. 

Carbon dioxide is only water soluble when pressure is maintained (like in your water pipes). In this dissolved state it is called carbonic acid H2CO3. Since it is a weak acid, when you measure water right from the tap without letting it sit out for a day or two, it will give you a false low pH reading. Once the the carbonic acid has a chance to off-gas into CO2, and escape into the air, you can only then measure the pH of your water and confidently obtain a realistic reading. The difference can pretty big too. (something along the lines of one half, to one and a half pH points is no small matter. If anyone cares this is the relation...as best as I can figure...

 

CO2 + H2O --> H2CO3

After that the carbonic acid reacts slightly (and reversibly) in the water to form a hydronium cation H3O-+ and the bicarbonate ion HCO3-

H2CO3 + H2O --> HCO3- + H3O+

This little tid bit may hopefully save you some head scratching and headache/frustration and resolve concerns about your media (or it's dust).

There is also a diurnal pH swing phenomena due to algae and CO2 respiration. At night algae use O2 and give off CO2 (which again is a mild acid, so it'll lower your pH reading)...during the day they use up much CO2 (which inversely will raise your pH reading)... Take a reading at dawn...and take a reading at dusk, and compare the two...

Between the above two suggestions, I'm willing to bet that you solve your pH woes/mysteries 

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