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TDS vs hardness

rr16

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536
Hi all,
I've not studied chemistry for some time and have not a great knowledge of TDS.

How does TDS compare as an indicator of suitable water vs the hardness of the water.

I have just invested in a TDS meter today and have tapwater with a TDS of 60ppm, but the water in my tanks was around 680, 420 and 320 ppm. The tank with 680 has wild A. panduro spawning in it and a KH of approx 1degree when taken with a JBL test kit and pH 5. The nitrites and ammonia are zero, and the nitrates are around 5ppm max. The phosphates are high however (I think due to the breakdown of so much leaf litter). So, I guess my question is, is it to do with the actual total ionic concentration (if that's the right word) of the solution, or is it more the concentration of particular ions in the water that matters?
 

gerald

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If CaCO3 and MgCO3 (the hardness salts) were the only ions present, then each 1 degree of GH or KH (~ 18 mg/L) should give you roughly 50 uS of conductivity, if I got my conductivity calculations right (someone please correct me if not). All those other ions that add to conductivity but don't affect hardness (Na, K, Cl, SO4, NO3, PO4, etc) may be building up from fish food, if not from your water supply or tank furnishings. Some people will say total ionic content is most important to fish; others will say that hardness ions Ca and Mg are what really matters. Either opinion might be right or wrong depending on which physiological process you're talking about (osmoregulation, egg fertilization, etc). Wish I knew more. There's a talk on this subject at the Raleigh NC aquarium conference in Feb; I will try and remember to post here on any useful things I learn there.
 

rr16

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536
Thanks, that's much appreciated. I imagined it would have something to do with whether you're looking at osmotic pressure and osmotic gradients, or different ions that may be involved in neurology or other physiological processes.
I'm not very clued up on it all - perhaps I should go and do an aquaculture course!
Also, I have used fertiliser in some tanks so I imagine some of the ions in that are adding to the conductivity. Would H+ ions be adding to the conductivity then? I imagine they would, but then I don't know. Would incresed free hydrogen ions (decreased pH) contribute to the conductivity?
What I do know is that I have wild A. panduro and wild Copella (either compta or eigenmanni) both breeding in a tank with around 1 degree hardness (carbonate), pH 5 and a TDS of around 680ppm. When people have talked about some of the more hard to get hold of species I've read then discussing having TDS less than about 40 to breed, yet I wonder how much it depends on the source of the ions contributing towards the TDS.
 

gerald

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No, I don't think so, because the total amount of H+ and OH- (actually the product of [H+] * [OH-] in water is always the same (10 -14 (that -14 is supposed to be an exponent) regardless of what else is dissolved in it. If you add acids to increase H+ , the OH- should decrease by a similar amount, so the conductivity caused by H+ and OH- ions together shouldn't really change. It's all those other ions that contribute most of the conductivity).
Would H+ ions be adding to the conductivity then? I imagine they would, but then I don't know. Would increased free hydrogen ions (decreased pH) contribute to the conductivity?


I'm sure it does depend on the particular mix of ions. Excess calcium can damage kidneys (in fish as well as humans) but it's not clear what role dietary calcium vs water-dissolved calcium plays, and how the ratio of Ca:Mg (in diet and water) affects all that. No doubt some ions can affect a fish's need for other ions, and also affect a fish's tolerance for high or low levels of other ions. Ya - go take that aquaculture class and then report back to us!
I have wild A. panduro and wild Copella (either compta or eigenmanni) both breeding in a tank with around 1 degree hardness (carbonate), pH 5 and a TDS of around 680ppm. When people have talked about some of the more hard to get hold of species I've read then discussing having TDS less than about 40 to breed, yet I wonder how much it depends on the source of the ions contributing towards the TDS.
 

rr16

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536
Ya - go take that aquaculture class and then report back to us!
Ah, if only I could afford it.
Also, I suspected something like that about the H+ and the OH- but I wasn't sure. I think I understood all this when I was 18, however it's all just gone out of my head now!
Also, physiology was never really my strong point!
 

wethumbs

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476
Don't underestimate yourself. I remember I hated electromagnetism from my freshmen general Physics classes, so I loathed the two junior required courses of electromagnetism. Through hard work, I aced both of them. Quantum mechanics was a different story though, that's what separate the men from the boys.
 

dw1305

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Location
Wiltshire UK
Hi all,
That is a really high TDS, my tanks run at about 100microS. (65ppm TDS). Have a look a this thread that discusses TDS <http://www.apistogramma.com/forum/threads/ok.12048/>.

As Gerald knows only too well, I've found pH an endlessly bewildering measurement, but there is a thread where "Regani" (who is a chemist) explains about H+ ions, self-ionization of water and pH <http://www.apistogramma.com/forum/threads/700-liter-amazone.13197/#post-71467> and I eventually I think I understand it.

Assuming I do, completely dissociated acids will raise conductivity by adding H+ ions.

cheers Darrel
 

gerald

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Keeping aquariums gives us a reason to learn and understand chemistry, physiology, physics, etc. it's way more fun and easier to learn and remember complex ideas when they're relevant to the things you really care about.

OK Darrel's right about the conductivity from H+ increasing when acid is added - yes, it does. My point ( 4 posts above) was that the H+ conductivity increase is offset by an equal decrease in OH- conductivity (Regani where are you, and is this correct)? So it's really the other ions besides H+ and OH- that contribute most of the rise in conductivity and have significant biological effects . I'm surprised some enterprising premium-aquarium-products marketer isn't yet offering bottles of pure stabilized H+ ions. |;>}
 

MickeM

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STOCKHOLM , SWEDEN
In this thread you can see pics of planted tanks..
If I remember it correctly, my friend told me that he once measured the Electric Conductivity (EC ?) in some of these tanks..
It then showed skyhigh results.. (way above recommended amounts!!) I don`t know about the TDS in these tanks..
However ..he got fry from several Apisto-/Tetra-/Killie fishes !!!
Maybe worth to think about when testing new strategies ..?

http://www.apistogramma.com/forum/threads/some-pics-from-a-basement-with-planted-tanks.17497/

/ Micke
 

regani

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Brisbane, Australia
I am right here Gerald, didn't you see me? :)

the H+ and OH- concentrations don't need to cancel each other out, if that was the case you could only ever have water with a pH of 7. you can have either more H+ or more OH- but together they will always add up to the same number. formula for that is:
pH + pOH = 14
where pH is the measure for the concentration of the hydrogen ions and pOH is the measure for the concentration of the hydroxide ions. so in neutral water both pH and pOH are 7.
when you increase the concentration of H+ the pH will go down (because pH is the negative log of the H+ concentration) and the pOH will go up because the concentration of OH- decreases (because pOH is the negative log of the OH- concentration)
you were probably thing of the fact that charges need to cancel each other out, that the overall charge of the water has to be zero - otherwise you'd be able to go to a bar and steal a glass of beer using a magnet :)

on the issue of conductivity...
as pointed out before, conductivity is only a substitute measure to give you an idea what is going on in your tank. it can only pick up conductivity changes due to salts or acids/bases and what exactly causes this change in conductivity is just an assumption on our part. in some cases those may be correct, in other cases they may not be.

the problem with TDS is that it is a really sh*tty value to measure and fairly useless in most circumstances. to correctly measure "total dissolved solids"
you will have to take a very precisely measured water sample and remove all the water - either by boiling it off (a bit old fashioned) or by freeze-drying, using a speedvac or similar methods. then you will need to accurately weigh the residue that is left over. that way you end up with a value that is usually expressed in mg per litre or ppm. (and a mg of something is not very much, you can hardly see it at all)
However, weighing out what is left over tells you nothing about what was actually in the water. it could be a mixture of salts, e.g. from sea water, it could be a whole bunch of sugar, e.g. from a soft drink, it could be a whole assortment of organic compounds, e.g. from a waste water sample. the samples all even could have the same TDS value, i.e. the same amount of 'stuff' in the water. so you could have a TDS of 1200 for all of them, but it tells you nothing about the water at all.

A value like "total dissolved solids" only really makes sense if you know what those solids are. an example where TDS is used is in aquaponics. here you usually deal with reasonably high concentration of fertilisers added to the water. as the fertilizers are various mineral salts, adding these will increase the conductivity. because conductivity and concentration of a salt or a specific salt mixture have a linear relationship over most of the applicable concentration range conductivity is a good substitute measurement under these specific circumstances. all you need to do is make up a few samples with your fertilizer mix at different concentrations in water and measure the conductivity. from there you can work out a 'correction factor' which will allow you to directly convert conductivity to TDS. e.g. you put 500mg of your fertilizer into 1L of water and measure a conductivity of 1500 uS. now you know that if you measure conductivity you just need to multiply by 3 to get the actual amount of fertilizer in your water. your test solutions will show that the values are more or less on a straight line for most of the concentration range so you know that that factor will work for the range of concentrations that are relevant for your hydroponics system.
the reason why this works in this case is because the concentration of fertilizers compared to anything else in the water is so large that whatever else may be in there has only a marginal effect on the measurement. there could be a small amount of other salts that make their way into the water or some organic matter that doesn't alter conductivity, but for the purpose of just checking your fertilizer concentrations they don't matter.
the commercial TDS meters will measure conductivity and have a built-in conversion factor so that you can read TDS - which gives a meaningful value for the test solutions that manufacturer used or for the intended application. in some of them you can even tweak that factor for different applications. but the specific value only has a meaning for an exactly defined system/application.
I wish people would stop talking TDS and used conductivity instead....

so we are back to the problem of how useful conductivity measurements are in a fish tank. the answer to that probably is: it depends :)
 

rr16

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5 Year Member
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536
So, now I bought a TDS meter, should I be buying a conductivity meter or can I make a direct conversion?
Or, infact, should I just be using test kits to check ammonia, nitrite, nitrate, phosphate, pH (my pH meter seems somewhat erratic, but then I have very soft water), etc?
I have wild A. panduro successfully raising fry and wild either C. compta or C. eigenmanni (or else some other random Copella species) spawning (and fry surviving) in my tank which has (I think, less than 1 degree KH - can't remember which degree but it's JBL so maybe German degrees), pH anywhere between 5 and 6, with TDS of 680, no ammonia, no nitrite and less than 5ppm nitrate, but high phosphate.
Will the high phosphate directly do any damage to the fish? From what I've read it's more causing algae etc that's an issue.
I perhaps should perform more water changes on this tank.
I guess my point is - are the other readings more important that give me an idea of what is in the tank (it also has lots of tannins from bogwood and leaf litter) as these species are breeding successfully so presumably that side of their physiology is OK?
I guess the real test would be to put some of the supposedly difficult to breed blackwater species in it and give it a try!
From what Regani is saying, I get the impression it is more what is actually in the water as opposed to how much of everything is in there, which is what I suspected originally.
I may just do a water change to reduce the phosphates a bit, but I'm also wary of doing too much due to having all those fry in there! Nice and slow, nice and slow!
Sorry, this post is a bit all over the place - don't think I'm fully awake yet!
 

regani

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You can make the direct conversion. The easiest way is to get a calibration solution for a conductivity meter so you know what the conductivity should read, then just figure out the conversion based on the reading on your TDS meter. Write down the reading that you see and for future calibration just tune back to tgat reading with the same calibration solution...
 

gerald

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Location
Wake Forest NC, USA
A "TDS meter" IS a conductivity meter. Cond is what is actually being measured. The display on the screen is shown as TDS because the meter is converting it. The conversion factor is usually either 0.50 or 0.65 (mg/L to uS ratio), depending on whether it's calibrated for solutions where sodium chloride is the predominant salt (.50) or for freshwater lakes and rivers with a typical mix of monovalent (+/-1) and divalent (+/-2) ions (0.65). The meter or packaging should tell you what the built-in conversion factor is. In other words, .50 gram of NaCl dissolved in a liter of water creates the same conductivity as .65 grams of "typical freshwater ion mix" (Na, K, Cl, Ca, Mg, SO4, HCO3, Fe, etc) in a liter of water. Na and Cl are predominant ions inside the fishes' cells, so even soft water species can tolerate quite a bit of NaCl in their water without ill effects.
 

rr16

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Mine says it's calibrated at 50ppm - think it's for testing drinking water.
 

gerald

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There's meter calibration for accuracy ("calibrated at 50 ppm") and there's calibration for different ion mixes (seawater, freshwater, chicken soup, etc) - two different (unrelated) kinds of calibration. I think the TDS/cond ratio (ion mix calibration) ought to be around 0.65 if it's designed for drinking water. But whether its .50 or .65 isnt all that crucial for fish keeping; you'll be in the right "ballpark" either way.
 

rr16

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So should I be doing the calculation TDS/0.65 = conductivity if TDS/conductivity = 0.65?
Have I got this the right way round?
 

dw1305

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5 Year Member
Messages
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Location
Wiltshire UK
Hi all,
Keeping aquariums gives us a reason to learn and understand chemistry, physiology, physics, etc......
This forum has helped me a lot.
I'm surprised some enterprising premium-aquarium-products marketer isn't yet offering bottles of pure stabilized H+ ions. |;>}
"......not just old fashioned dihydrogen oxide, and exclusive to us, our all new 21st century water conditioner has added proton power....".
So should I be doing the calculation TDS/0.65 = conductivity if TDS/conductivity = 0.65?
Have I got this the right way round?
All the conductivity meters I've used in the UK have had 0.64 as the conversion factor, so 100 microS conductivity ~ 64 ppm TDS.
infact, should I just be using test kits to check ammonia, nitrite, nitrate, phosphate, pH (my pH meter seems somewhat erratic, but then I have very soft water), etc? I have wild A. panduro successfully raising fry and wild either C. compta or C. eigenmanni (or else some other random Copella species) spawning (and fry surviving) in my tank which has Will the high phosphate directly do any damage to the fish?
No, there is no real point in using test kits. There would be a point if they gave you accurate repeatable values, but for a lot of parameters they don't. Anions, like NO3-, are particularly problematic.

Your pH meter should give you a fairly stable value, because your water has a high conductivity. One thing about pH meters are that they are fairly high maintenance equipment. You really need to buffer them at pH4 and pH7 before every use. You also need to store the electrode in the correct storage solution, and remember that, even when properly stored, the electrodes have a limited life span.

Your phosphate PO4--- (as hydrogen phosphate (HSO4)--) won't contribute much to the total conductivity. Drinking water doesn't have a "safe limit" for phosphate in UK, mainly because it doesn't cause any health problems at high levels. All the water companies add orthophosphate to their water supply at 1-3 ppm (and if your water is soft NaOH as well), the reason for this is to raise the pH, and take any lead, zinc or copper ions out of solution. The heavy metal ions will form <"insoluble phosphates and be precipitated out of solution">. Phosphates are essential for plant growth, and whilst it is true that they are a major player (along with nitrogen) in the eutrophication of rivers etc., in the aquarium they don't really "cause algae" or damage your fish.

I would increase the number of water changes, but I think the most important fact is that your fish are thriving, and that shows there isn't a lot wrong.

cheers Darrel
 

rr16

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536
Much obliged! I've used pH meters in the lab before, many years ago and often remember calibrating them using the pink and green pH 4 and 9 ( I think!) solutions to calibrate them every time when I was making PBS or other random buffers for SDS PAGE or ELISA.
 

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