Hi all,
I am well out of my depth now so Mac or DW can answer the other bit.
No, I don't need to add anything, but I can explain the scientific bit.
Lastly since there is nothing added, is the buffering a problem, will my pH vary?
It certainly will as you approach 0KH, at this point pH is a meaningless measurement. Have a look at this post: <
http://www.plecoplanet.com/forum/showthread.php?t=6901> (and links in it) for an explanation of why (it contains a bit of chemistry, but unfortunately there is no way around that).
Water
Bob and G have "Bristol Water" water, which is coming from either the Sharpness canal (River Severn) or most likely the Chew & Blagdon lakes. All of this water is naturally carbonate (limestone) rich (has a high KH), and also contains an amount of nutrients - NPK etc. (from agriculture mainly, it is "eutrophic" to use the technical term).
I visited the Bristol Water site and got this print out for "BS14".
Supply zone: Hengrove and Whitchurch <
http://www.bristol-water.co.uk/environment/waterQualityInYourArea.asp>
pH pH 7.57
Alkalinity mg/l CaCO3 157
mg/l HOC3 191
Calcium mg/l Ca 94
Magnesium mg/l Mg 6.5
Total Hardness mg/l CaCO3 262
Degrees Hardness
Clark° (UK) 18
French° 26
German° 16 (dKH)
The important bits is the hardness 16dKH, virtually all from CaCO3.
Full report is here: <
http://www.bristol-water.co.uk/pdf/environment/waterQuality/220.pdf>.
So this is their original water and as they explained the RO and HMA filters then do different things.
RO - (Reverse Osmosis) Filter
The 3 stage RO unit removes everything from the water and you are left with pure H2O (so just H+ and OH- ions in a ratio of 1:1). Pure H2O is an electrical insulator, so if we pass an electrical current through it (which is what the TDS meter does) none will arrive at the meter to be measured. The technical explanation is
"Conductivity is the ability of a material to conduct electric current. The principle by which instruments measure conductivity is simple - two plates are placed in the sample, a potential is applied across the plates (normally a sine wave voltage), and the current is measured. Conductivity (G), the inverse of resistivity (R) is determined from the voltage and current values according to Ohm's law.
G = I/R = I (amps) / E (volts)
Since the charge on ions in solution facilities the conductance of electrical current, the conductivity of a solution is proportional to its ion concentration.
A bit technical, but a picture is better than a thousand words, and this is what that means:
As you can see the relationship is linear for salts and slightly more complex for strong acids, but for weak acids, like the humic acids from Indian almond leaves, peat or alder cones we can assume it will be a straight line as well.
The neat RO has no buffering and the addition of any acid, however weak will cause the pH to fall, potentially to very low levels. Exactly the same would apply to any alkali it will cause the pH to rise in the same manner. If we had a situation where we had changing levels of CO2 and O2 (in a planted tank perhaps) the pH could fluctuate wildly from acid to alkaline.
If you ignore the pH readings (this was for salt water) this graph shows what I mean, the high photosynthesis rate of the plankton depletes the CO2 and saturates the water with O2 and the pH rises, at night CO2 levels rise from respiration and no O2 is evolved and the pH falls:
.
HMA - (Heavy Metal Axe) filter.
To get around these buffering problems you need a source of KH, Bob and G get this from their naturally hard "Bristol Water" tap water, the reason for the HMA filter is that it then removes both nutrients (NPK etc) and pollutants (heavy metals, chlorine, pesticide traces) from the water,
but it what it doesn't do is remove or alter the hardness.
All you have to do then is get a mix of HMA and RO water that your fish thrive in, and measure the TDS. In this case we know that the TDS is largely a measure of the dissolved calcium carbonate (because we've removed everything else with the 2 filters). Add the needle valves and you can get any water you want by changing the ratio of RO:HMA, and if you want soft, low pH, low conductivity water you can add some weak humic acids from Indian Almond leaves etc. Sheer genius.
The only other thing to note is that:
If you have different tap water with low levels of carbonates (hardness) their method will not work in the same way, and you will need to use RO buffering salts (Na2OH, shell grit, MgCO3 etc.) to add the necessary KH .
cheers Darrel
