substrate buffering ... what does it mean?

[AussieTanker]

i think this is a substrate topic rather than water paramerters ... so i hope that i got that right so far ...

i am planning on redoing my 30gall tall .... i want to replace my current substrate which is white and has too large a particle size .... i am wanting to use a darker substrate and as i can't get eco complete here in aussie am thinking of using seachems black onyx sand ... which i can get ...

i have been told that it will "buffer" the water ..... in am not sure what that means ... is it a good thing ? ... ( this substrate is meant to be especially made for plants ) ...... can someone pls tell me


i have also been told that it will raise ph ... my ph out of tap is about 8.2 .... and very soft ... i have pressurized co2 ... which brings it down to 6.8 .... what practical effect will using onyx sand have on my tank and its water parameters .... will i need to use more co2 to bring the ph down ... or does it mean that the ph will swing wildly ...


your urgent advice is greatly apppreciated ....

regards
aussietanker

[ninoboy]

I use Onyz sand in two of my tanks. The buffering is minimal and I don't even get any significant change on the parameter reading. At the Seachem website, it says that this substrate may bring up the pH between 0.1-0.5. I used 3 bags in one of my tanks and maybe only get as much as 0.1 increment on pH.

[malkore]

buffering means it's going to increase your Kh (carbonate hardness), but I think you only get 1-2 degrees out of it.

You say you have soft water...but a tap pH of 8.2, which to me means a high alkalinity/carbonate hardness.

you should let the tap water sit in a bucket for 24 hours, then retest the pH and see if it's still 8.2 and better yet, list your actual Kh.

You can grow plants very well in plain gravel, and if you don't need the Onyx for added buffering (and you probably don't) then it may not be worth the expense...unless you like the color.

[AussieTanker]

tjhanks for the reply malkore .... its appreciated

yes, im sure that the figures are correct ... but will check again on the water board website ...... it always tests that high with my kits ... even after "sitting"for some time ..

sydney aust has very alkaline water ... ph around 8.0 - 8.4 out of the tap ... but also very soft water with a total hardness (gh) of about 4gh ..... and a carbonate hardness of about ( 3Kh) .....

i dont think high ph always = high hardness .. but then again im not sure .... im a biit confused now

the main reason that i want the onysz is for the colour .. and its inbuilt plant nutrient factors .... we cant get eco complete here in aust ... and it seems to be the next best thing ... ( in fact ... so far as "special "plant susbstrates go .... its our ONLY thing ,... there are no other choices other than very light or brown gravels or sands ... )

can anyone clarify this issue a little more pls .. are ph and kh linked somehow? .... will using onyx sand increase my ph more .... and how will that affect co2 ( pressurized system) ... will it cause wild swings ?

thanks for helping

regards

[malkore]

Kh and pH are linked. As carbonate hardness increases, alkalinity increases, so pH has to increase as well.

This is how we're able to use the Kh/pH chart to determine our CO2 levels....without a strong phosphate buffer, Kh & pH should be proportionate to each other. Adding CO2 creates carbonic acid which lowers pH, but leaves Kh untouched...this skewing of the Kh/pH relationship is how we see that a Kh of 8 degrees and a pH of 7.0 yields a CO2 level of roughly 25ppm - ideal level of CO2.

That's whats strange about your water...3d Kh is the bare minimum recommened Kh level if you're going to inject CO2. At 3dKh I'd expect a tap pH of only 7.4 (my Kh is 8degrees, pH 7.8)

I suggest you test your tap water after it has sat out for 24 hours in a bucket with an airstone running in it.


Onyx would increase your pH and Kh, but only by about .2 and 1 degree respectively.

[AussieTanker]

hi malkore,

thanks again for your persistence .... your explanation makes sense to me, and haas helped a lot in my understanding ... ... i will try to repeat the test after letting the water sit for a few days ...

i have checked with the water board web site but dont quite understand the figures ... they say the pH is 7.8 - 8.2 ... with a total hardness of 49-56mg/L CaCO2 .... they give no figure for carbonate hardness ....

im not sure if that figure can be converted direct to degrees hardness ... or if it even makes any sense to you ... or whether it points to an error in my testing/test kit ?

regards
aussietanker

[malkore]

Total hardness is most likely ONLY referring to General hardness...unless they're averaging Gh and Kh together. 56mg/L isn't much...about 3 degrees of hardness.