Sand's affect on ph?

I have a nearly 5 yr old 90 gal reef that has prospered relatively well. But I want it to do better. One issue I have never been able to resolve is to stabilize my ph where I want it (8.1-8.3). It sits around 7.7, no matter how much I buffer it. It raises a little after buffering but quickly falls back to around 7.7. I have used many different test kits over the years and get the same results. I have been researching this issue and came across two interesting articles on the Seachem website by Leo G. Morin, Ph.D. It has to do with substrate selection. After reading these articles, I'm considering the arduous task of replacing my crushed coral with sand. Below is an excert from each article and the link to the full articles after each excert. I'm just curious about anyone elses thoughts/opinions on this topic.

The substrate for the marine or reef aquarium must be
mostly insoluble under normal aquarium conditions. One of the
better and more economical substrates is crushed oyster shell
(used in poultry feed). Dolomite is suitable. Other crushed shells
and coral sand are also suitable. Soluble or so-called “buffering”
substrates such as crushed coral should be avoided, because
such substrates release into the confines of the tank an uncontrolled
quantity of minerals, causing a pH drop and an inability to​

maintain pH, ultimately buffering at 7.6.

http://www.seachem.com/Library/SeaGrams/Marine_Conditioning.pdf

It is evident from our experience and the understanding of buffers
described above that the addition of calcium ions in the form
of carbonates (lime, crushed coral) will ultimately lead to a pK
drop, and ultimately, to a buffer at pH 7.6. This experience and
the consequent conclusions have been confirmed by several
hobbyists and professional aquarists. For the same reasons, the
use of bicarbonate or carbonate to raise pH only increases the
capacity of the buffer and temporarily raises pH, but does not​

significantly raise the pK of the buffer.


http://www.seachem.com/Library/Articles/Marine_Water_pH_Control.pdf

 

i agree.
the crushed coral will constantly release carbonate into the surroundings until it is at equilibrium with them. this results in a variable carbonate level. each time you add buffer it does its job, however it is easily combated by the crushed coral, thus a pH at the lower level.
The only way to surely raise it is to make your pK go up so it is buffered at a level near 8.2. with the carbonate concentration constantly being released from your CC its just not going to happen

 

i think i have this same problem. i don't have crushed coral though i have bleached coral skeletons in the tank for base rock. would this have the same effect? how can u increase pK?

 

to increase pK you have to take out your coral. it is because the concentration of CaCO3 in the skeleton is high. as it leeches out to bring the tank to equilibrium with it, a Hydrogen dissociates and forms H2CO3 and this is a weak acid. this is the acid that buffers you at 7.6-7.7

 

this can be adjusted in more ways than one. do you have your tank parameters and ill see what i can do to fine tune your pK?

 

Have you ever checked your DKH? If you believe that your issue is your crushed coral, what do you think is a primary component of liverock? We should investigate other potential issues that could be causing a low PH level. Crushed coral is notorious for decreasing PH due to it's ability to trap large amounts of detritus, and the slow decay of detritus can lower your PH as well.

 

Your problem can also be too much Co2 in your house. I used to have the same problem, ph never going any higher than 7.9
Try this little experiment......
Take a glass of tank water and place an air pump with an air stone in it and run the pump for about 20 mins. Test the ph and write down your result. Take that same glass of tank water and go outside and do the same thing. Test your ph again and post your results.
BTW, I hope that you dont have a cover over your tank.


Luna

 

yes, two CO3 from above rxn dissociate an Oxygen each to make O2 and CO2 in the presence of a (base)buffer. the O2 evaporates into your house if your house has higher CO2 than outside to try and stabilize the atmosphere in your house. this leaves the excess CO2 in your tank as Luna said

 

HEY!! I remember writing an article about this a while ago! ;D

http://www.3reef.com/forums/water-chemistry/ph-rant-56815.html#post537740

 

Thanks so much for your help.

I'm running the test Luna suggested now. I'll post the results when I'm finished. I do not have a cover on the tank. In fact, one of my k3's and both of my returns are pointed up to increase surface agitation/oxygen exchange. In the mean time...here is the latest on my other parameters.

PH - 7.7
KH - 10
No3 - <12.5
MG - 1335
Ca - 415
SG - 1.024
Temp - 77-78