Difference between TDS, conductivity, kH and gH?

Mei · Post by **Mei** » 05 Apr 2006, 03:47

Hi,

Although this is my first post on this forum, I have been following the site for quite a few years and have found such a wealth of knowledge in your posts. If someone could help me out with the following, it would be much appreciated.

I am often confused by the use of the terms TDS, conductivity, kH and gH. I am currently using test kits for kH and gH. Am I able to use the results of these tests as guidelines to how my tds / conductivity is fluctuating? How do these terms relate to each other?

Thanks,

Mei.

Post by **Barbie** » 05 Apr 2006, 05:05

Mostly these terms just related to the hardness of the water and the method of testing it. TDS is a meter that measures the total dissolved solides. Conductivity is another type of meter that measures how well the water carries current (more dissolved solids, more current). Carbonate hardness, or the waters ability to maintain a pH (also known as buffering capacity) is the kH. General Hardness is gH, which is basically all the other dissolved solids that aren't directly impacting the buffering capacity (such as salt and what not). Did I clear it up at all or just muddy it up more for you? ;)

Barbie

CatBrat · Post by **CatBrat** » 05 Apr 2006, 06:20

How do you measure conductivity and TDS?

Cheers, CatBrat.

Mei · Post by **Mei** » 05 Apr 2006, 08:27

Muddy? No, it's more clear.

Thanks for that. I've just been trying to compare the measurements on this site (for triggering spawns) with the conditions in my own tanks. I've been breeding the common plecos that we have here in Aust, on and off. But have decided to take a closer look at the effects of each parameter.

I've been using gH and kH as a gauge to the level of TDS in the tanks but I've now decided to bite the bullet and purchase a TDS meter. It's not too expensive after all. CatBrat, this is the one I will be buying at, "http://www.psifilters.com.au/Testing%20Meters.htm".

I've been running a dry season in my tank for almost a week and haven't seen an increase in gH or kH. I was just curious to know whether the TDS had increased.

Thanks again.

Mei.

Mike_Noren · Post by **Mike_Noren** » 05 Apr 2006, 11:04

Mei wrote:I am often confused by the use of the terms TDS, conductivity, kH and gH. I am currently using test kits for kH and gH. Am I able to use the results of these tests as guidelines to how my tds / conductivity is fluctuating? How do these terms relate to each other?

OK, brace for impact...

1) TDS and conductivity are the same thing, but using different scales.
Conductivity is a measure of how well electricity passes through water, and that is an indirect measure of the amount of electrically charged ions in the water, ie the salt content. Rain water has very low conductivity, while the conductivity of seawater very high.
TDS stands for "total dissolved solids", but that is a misnomer, it does not measure all dissolved solids, but measures the exact same thing conductivity does, but recalculated and expressed as concentration of a standard salt mix.

Conductivity (and TDS) can only be measured with electronic meters, and is independent of pH, KH and GH.

For some reason people often question that TDS is exactly the same thing as conductivity only the measurement has been recalculated as concentration of standard salt solution, so I will refer to this: http://www.reefkeeping.com/issues/2004- ... /index.php

2) KH is the concentration of bicarbonate in your water. Under the assumption that the only pH-buffering compound in the water is bicarbonate, KH is generally conflated with alkalinity (pH buffering capacity), but in aquaria this assumption frequently does not hold, due to the use of e.g. phosphate buffers.
Luckily, the interesting thing is alkalinity, not KH, and alkalinity, not KH, is in fact what KH tests measure.

3) GH is the concentration of ions in the water with +2 electric charge, e.g. calcium and magnesium ions, recalculated and expressed as milligram limestone (calcium carbonate) per volume. As far as I can gather, the reason for this bizarre recalculation, is that this measure was originally used to determine how much limestone scale a particular water would cause in steam engines and boilers. I see very little, if any, use for GH in aquaristics.

Now, for the good news: most of the time you do not need to worry about any of these measurements, or pH for that matter. Most of the time, the fish simply doesn't care. Really.
The exception is if you're trying to BREED the fish. Quite a lot of fish use changes in conductivity and temperature as a trigger to breed, as conductivity and temperature will both drop at the start of the rainy season. There's been a lot written about this, I refer you to the breeding articles here on Planetcatfish for details.

Other than that: acclimatise new fish slowly, keep your water parameters, whatever they are, steady, and your fish will be happy.

bronzefry · Post by **bronzefry** » 05 Apr 2006, 13:18

Mike,
Do you mind if we use your definition of conductivity in the glossary?
Amanda

housewren · Post by **housewren** » 05 Apr 2006, 13:47

Mike,
Are you saying that I can KEEP (not breed) most South American fish (neutral to acid pH, soft water) just fine in my hard, alkaline (pH 8.4-8.6, dKH 18, dGH 20) just fine as long as these parameters are stable?

I have shied away from most of them (sticking primarily to livebearers and Rift Lake species), except for in two specially set up tanks, because I can't store enough rainwater. I would love to be able to keep Kribs, Rams, etc. but thought I would have trouble.

Cheri

Mike_Noren · Post by **Mike_Noren** » 05 Apr 2006, 13:52

Feel free to use it, but I'm nobody. I would suggest also providing a link to a more authoritative source, e.g. Randy Holmes-Farleys in-depth treatment here: http://www.aquariumfish.com/aquariumfis ... 24&search=
It's about conductivity in salt water, but there is really no difference except the range.

Housewren: Yes, that is exactly what I'm saying. And in fact a lot of people do.

Post by **Barbie** » 05 Apr 2006, 19:58

Just keep in mind that pH is a factor in how toxic ammonia will be to the fish, should you have a problem with your filtration. Ammonia is 10 times more toxic to your fish at a pH of 8.0 than it would be at 7.0. As long as you account for that and keep your water parameters stable, the fish usually don't have any problem once they've been properly acclimated. There are a few exceptions to this rule, but not as many as people think.

Barbie

Post by **Janne** » 05 Apr 2006, 20:46

I see very little, if any, use for GH in aquaristics.

It can be useful for aquaristics for them that not have the money or possibilitys to buy a conductivity meter, you can estimate the conductivity when measure the dGH...dGH 1.0 ~ 40-50 ÂµS depending on what type of salts but in tapwater there are mostly calcium and magnesium present and very little of the other (with a few exceptions).

Janne

bronzefry · Post by **bronzefry** » 05 Apr 2006, 21:03

Thanks for the link, Mike.

Amanda

Mei · Post by **Mei** » 06 Apr 2006, 09:26

Informative. Thanks guys.

Mei.

Jackster · Post by **Jackster** » 06 Apr 2006, 17:16

I have been able to keep quite a few soft water species in my very hard tap water.
I have also been told by others that Discus and Angel fish can even withstand
my water parameters but most in my area of Wiscosin manage to not have very good
results with Discus, Angels, Apistogramma, and Mikrogeophagus (Rams) without the
help of R/O water. I find that many Loricariidae do fine in my tanks but there are
some that don't.

My tap water is 8.3 pH 24Â° dh GH and 8Â°-12Â° dh KH and is considered very hard.
I will disagree a little with the definition of the above for KH. KH is the measurment of
carbonate CO3(2-) and bicarbonate HCO3(-) anions (or negatively charged ions).
KH is usually a good enough test to use for monitoring buffering capacity but several
other anions can affect buffering which include sulfate, hydroxide, borates, silicates,
and phosphates.

Post by **Shane** » 08 Apr 2006, 00:46

dGH 1.0 ~ 40-50 ÂµS depending on what type of salts

This shows how adaptable our fish are and how hard it is to replicate their natural environments. In the hobby 1 dGH would be considered very, very soft water. Look at these figures
Rio Trombetas 20 uS
Rio Abacaxis 17 uS
Rio Urubu 16 uS
Rio Branco 23 uS
upriver Rio Negro 11 uS
Main Amazon 85 uS (near Benjamin Constant, white water, dry season)
Rio Nanai 6 uS

The truth is that, for many Amazonian fishes, we should be shooting for water around .15-.30 dGH. At the "worst" part of the year (high, dry season) the fish are temporarily exposed to water around 1.75 dGH. In the hobby, 1.75 dGH would still be considered very, very soft water!
-Shane

Mike_Noren · Post by **Mike_Noren** » 08 Apr 2006, 10:38

Unfortunately dGH is not easily translatable to conductivity. For instance, table salt will change conductivity but not dGH at all (although the chloride may poison the reagents in the dGH test so it will give bizarre readings).
dGH is simply a measure of ions with a +2 charge, so in natural waters it is mainly a measure of calcium and magnesium, while it ignores e.g. sodium and potassium.

Now, you may be lucky and live in an area where the proportion of ions in the water is about the same as in the area of origin of your fish, but I'd say that for most of us that does not hold true.

EDIT: I should also add that one thing to be very, very, careful with, is moving a fish from a water with high conductivity (as in an LFS tanks) to an aquarium with low conductivity (as in a conscientous hobbyists RO-water tank). The result will be that water rushes in to the fishes body, cells swell and rupture, the body fluids get diluted - and the fish may die. Slow acclimatization is needed.
Going in the other direction, from low conductivity to high, will generally not kill the fish, unless the salinity of the water is so high it leads to renal failure (basically only a risk if you've got a brackish water tank).

racoll · Post by **racoll** » 08 Apr 2006, 11:54

There's a very useful conductivity converter on the PFK website.

http://www.practicalfishkeeping.co.uk/p ... ertors.php

You will have to register to use it, but it's well worth it, as there's loads of useful stuff on there.

ndoboi · Post by **ndoboi** » 08 Apr 2006, 12:05

This thread is proving to be a very informative read

Are there tables anywhere that list what sort of TDS we should be aiming for when trying to breed different L#s?

Sorry Shane, im not familiar with the scale 'uS'. Is this the measurement a TDS meter will give us?

Thanks
Steve

Post by **Janne** » 08 Apr 2006, 12:21

Unfortunately dGH is not easily translatable to conductivity. For instance, table salt will change conductivity but not dGH at all (although the chloride may poison the reagents in the dGH test so it will give bizarre readings).
dGH is simply a measure of ions with a +2 charge, so in natural waters it is mainly a measure of calcium and magnesium, while it ignores e.g. sodium and potassium.

but in tapwater there are mostly calcium and magnesium present and very little of the other (with a few exceptions).

Thats true, thats why I said tapwater...not the extra salt we sometimes use in the aquarium. What I meant with a few exceptions was that there are some places that have high contents of sodium or natrium ions in their tapwater, potassium should not be any bigger problem when that are usually very low concentrations. Even the amount of natriumhydroxide they use to bring the pH up in our tapwater are very low concentrations and fit in that range I mentioned dGH 1.0 ~ 40-50 ÂµS...but no rules without exceptions.

Janne

racoll · Post by **racoll** » 08 Apr 2006, 12:22

im not familiar with the scale 'uS'. Is this the measurement a TDS meter will give us?

uS is actually μS.

It is used as μS/cm, and means microsiemens per centimetre. A microsiemen is a very small unit of electrical current.

This is the measure of conductivity (ie, how much electicticity can pass though a set distance of water).

This has a direct relationship to how much minerals are in your water, so it is converted into TDS by some sum.

TDS meters will usually be able to give readings in both μS/cm and TDS.

bronzefry · Post by **bronzefry** » 09 Apr 2006, 19:01

I should also add that one thing to be very, very, careful with, is moving a fish from a water with high conductivity (as in an LFS tanks) to an aquarium with low conductivity (as in a conscientous hobbyists RO-water tank). The result will be that water rushes in to the fishes body, cells swell and rupture, the body fluids get diluted - and the fish may die. Slow acclimatization is needed.

I had some quick losses once I started using RO water. I asked around and heard the same thing. I've gotten into the habit of asking what the water parameters are when purchasing a fish ever since. I often receive blank stares in return or silence at the other end of the phone. I also ask if salt has been added to their tanks or if marine fish are kept nearby. I've learned to test the water in the bag no matter what and acclimate longer if needed because of the RO water. What's an extra day or two in quarantine?
Amanda

ndoboi · Post by **ndoboi** » 15 Apr 2006, 04:37

Ok, my TDS meter arrived the other day and I have been shocked to discover what my water parameters really were....

Tap water - 360ppm
RO output water - 23ppm
Tank after daily 15% RO water for 7 days - 560ppm. WTF.

Did some further testing and have found that the likely culprit is Seachem ph7.0 Neutral Regulator. Hmm, the bottle says that it adjust a high, or low, ph to 7.0 and also "softens water by precipitating calcium and magnesium". So why the increase in TDS?? I tried using a small sample of tap water and it quickly went from ph7.5 to ph7.0 and TDS of 360ppm to 760ppm.

Has anybody else encountered this? Now I know one reason why this cycle hasnt triggered the catties....

Cheers
Steve

Mike_Noren · Post by **Mike_Noren** » 15 Apr 2006, 10:03

ndoboi wrote:Did some further testing and have found that the likely culprit is Seachem ph7.0 Neutral Regulator. Hmm, the bottle says that it adjust a high, or low, ph to 7.0 and also "softens water by precipitating calcium and magnesium". So why the increase in TDS??

Because hardness is not directly coupled to TDS, and that normal ratio of ions Janne talked about, which under certain circumstances allow you to estimate conductivity from hardness, goes right out the window when you start adding buffer.

The buffer consists of ions, and so raises conductivity, and hence also TDS (as TDS is the same thing as conductivity only recalculated as concentration of a standard salt mix) but as it contains no bicarbonate, magnesium, or calcium, it doesn't raise hardness. It will, however, APPEAR to raise KH, as the KH test in fact measures alkalinity, not carbonate hardness, and the buffer does raise alkalinity.

racoll · Post by **racoll** » 15 Apr 2006, 12:17

I would stay clear of those kind of products. I don't see any need for them. As you have discovered, they confuse what is already complicated enough!

bronzefry · Post by **bronzefry** » 15 Apr 2006, 14:46

Tap water - 360ppm
RO output water - 23ppm
Tank after daily 15% RO water for 7 days - 560ppm. WTF.

Quite a shock to the system, eh? The the products you mention do raise the ppm quite a bit. Barbie's method of adding a tad of tap water to RO water to reach the desired ppm is the best method I've used. Do you have a see-through canister on your RO filter? You may want to check the pre-filters. They don't last as long as the manufacturer says they do.

Amanda

Mike_Noren · Post by **Mike_Noren** » 15 Apr 2006, 19:01

racoll wrote:I would stay clear of those kind of products. I don't see any need for them.

Well, the *need* may be doubtful, but the *use* is clear: buffers prevent pH swings/crashes, which is otherwise a real risk with very soft (and hence almost unbuffered) water. That buffers doesn't change GH (as you haven't added any calcium or magnesium ions), do show up in "KH" titrations (as you've increased the waters ability to resist acidification) and conductivity measurements (as you've added salts), is all as expected.

racoll · Post by **racoll** » 16 Apr 2006, 09:51

Yes, absolutely.

I was in no way suggesting that one does not need to buffer the water, what I meant was that it would be easier to use a regular carbonate/bicarbonate buffering system, than a phosphate buffer that claims to "soften" your water by adding more salts.

Jools, as I keep getting involved in these kind of posts, I would very much like to write an article for Shane's World entitled something like....

"Water Chemistry and the Catfish"

ndoboi · Post by **ndoboi** » 16 Apr 2006, 10:50

What a fantastic idea!

blueguava · Post by **blueguava** » 18 Apr 2006, 02:50

Is there an agreed way of raising TDS in a tank after it has been put through a "wet season" without the nitrate increasing to much?

I don't have to worry about it as the TDS in my normal tap water is fairly high, but I have been asked the question by some friends that have very soft water from the tap.

Regards

racoll · Post by **racoll** » 18 Apr 2006, 09:38

Is there an agreed way of raising TDS in a tank after it has been put through a "wet season" without the nitrate increasing to much?

"RO Right" by Kent Marine is a basically powdered GH (Ca++, Mg++ & trace minerals etc).

Gradually adding a solution of this to the tank will raise TDS.

Mike_Noren · Post by **Mike_Noren** » 18 Apr 2006, 10:48

ANY salt will raise conductivity, and hence TDS.
Personally I'd simply do water changes with tapwater to raise TDS. If the tapwater wasn't suitable, I'd use bicarbonate to raise pH to the required level, then add table salt until the conductivity was where I wanted it.

Difference between TDS, conductivity, kH and gH?

Difference between TDS, conductivity, kH and gH?

Re: Difference between TDS, conductivity, kH and gH?

Raising TDS