Total Dissolved Solids (TDS)

TOTAL DISSOLVED SOLIDS (TDS)

I’ll try to get this across using not too much science….but some science will be needed. A complete scientific explanation would need more text and maybe some diagrams.

If anything needs expanding, the please ask.

TDS
This is the amount of solids dissolved or ‘gelled’ in solution that will not be filtered by a filter size of 2 micrometers.

If the solids can be filtered by a 2 micrometer filter or larger pore size then they are not part of the Total Dissolved Solids.

So, salts, non-volatile acids or bases, sugars, or any other inorganic or organic solid dissolved in water can contribute to the TDS.

Mud, peat, branches, sand, mulm etc are not part of the TDS.


In short and simple terms: TDS is a measure of the unfilterable solid chemicals impurities in water.

MEASUREMENT:

The most accurate method is to filter the water through a 2 micrometer filter, then evaporate all water and carefully measure all solids.
That requires laboratory quality balances and is not as easy to do as it sounds.

SO….we turn to the TDS meter for aquatic use.

TDS METER.

The TDS reported by an electronic TDS meter is not necessarily the TDS of water.
It is only an estimate, and would only be close to accurate in a few rare occasions.

Ummm? Why?

The electronic TDS meter is actually a conductivity meter made to give only an estimate of the TDS. But the accuracy is still only an assumption and has a built-in ‘assumed’ conversion factor.
Ie forget about accuracy with a TDS meter…..accuracy has little meaning with them, but they still have a use in determining changes or relative Dissolved Solids..

A conductivity meter measures how well the water will conduct electricity (in crude terms).

Addition of certain chemicals at certain concentrations will affect the conductivity.

However, different chemicals have different effects on conductivity.

Soluble salts such as sodium chloride, calcium chloride, sodium carbonate, iron chloride contribute to quite a great extent to conductivity.

But many other dissolved solids have relatively little or no significant effect on conductivity especially when compared to the salts named above.

But, you may also get chemicals that are not dissolved solids affecting the conductivity. But that gets into a more complex realm.

Hence, those compounds that do not contribute to any great extent to conductivity would be effectively ‘ignored’ by a TDS meter (remember it is a conductivity mere in disguise) even though they are Dissolved Solids.

That is why a TDS meter is not a true measure of Total Dissolved Solids.

DO YOU NEED ACCURACY WITH A TDS METER?

As said, accuracy and TDS meters should not be in the same sentence.

There is really no need to be concerned if your TDS meter is an accurate reading of actual TDS or not.

What is more important is that the TDS meter is used to detect changes in the water, and getting a rough estimate of something being ‘wrong’ is more important than quoting a TDS ppm results.

Different meters have a different calibration conversion factor (see below).
It is useful to know which factor is used, but not vital.
Two different brands of TDS meter may well give totally different results on the same sample of water.

CONVERSION FACTORS (this gets complex)

The electronic TDS meter will measure the conductivity, and then converts the reading into parts per million (ppm = mg/l) TDS by an internal conversion factor.

The conductivity is measured as microsiemens per centimetre (uS/cm)

The conversion from conductivity to TDS reading depends on what particular compound is dissolved.

Below are a few examples of Conversion Factors.

Sodium Chloride
TDS (ppm) is approx 0.51 the Conductivity (uS/cm).

Calcium Chloride
TDS (ppm) is approx 0.47 the conductivity (uS/cm)

Sodium Bicarbonate
TDS (ppm) is approx 0.97 the conductivity (uS/cm)


The problem is is that aquarium water is complex and contains different dissolved solids.

If the TDS meter is calibrated for sodium chloride, then a reading indicating 60ppm TDS would be 60ppm if the water is mainly a sodium chloride water, but if the water were mainly sodium bicarbonate then a TDS reading of 60ppm would mean there is about 120 ppm sodium bicarbonate. {in fact, it is not as simple as that as the measured TDS is not linear with the actual TDS….see below}

Hence, cross comparing different water-types has little meaning.

Some TDS meters allow calibration against a known standard solution.

Often, we see a conversion factor quoted of 0.67 to convert conductivity to TDS…..and divide TDS by 0.67 to get conductivity.
That value of 0.67 (or 0.7) is somehow picked out of thin air as an median approximation of conversion factors somewhere between approximating natural freshwater.

The conversion value of the 442 calibration system (40% sodium sulphate: 40% sodium bicarbonate: 20% sodium chloride) tends to approximate to a conversion factor 0.67.

ADVANCED CONSIDERATIONS for TDS METERS.

The size and hydration of ions affects conductivity.

Different compounds have different molecular masses….conductivity doesn’t really measure a mass, yet TDS is a measure of mass.

Temperature affects conductivity and thus affects TDS meter readings.

pH will affect the reading.

Some chemicals increase conductivity as more is added but then when even more is added the conductivity decreases. So….in these cases, the total dissolved solids are increasing but the TDS meter will show the TDS as decreasing. This is a problem of using TDS meters in concentrated solutions.

Selection of the right calibration solution may give better results for a particular water system.

ian

Temperature also affects solubility!
What dissolves at 2degrees is not what dissolves at 30.

murph wrote:

Temperature also affects solubility!
What dissolves at 2degrees is not what dissolves at 30.

That's part 2.

Temperature, concentration, localised colloids & surface electrical potentials and loads of other things come into play in part 2.

ian

Good read so far can't wait for part 2, but of what use is tds readings, what is optimal tds for let's say a malawi setup or an amazon setup and how do you attain these parameters if they are off optimum levels and again for non technical people like me pls make it idiot proof

Glad it is semi-readable at least.

As Murph, who I know has a background in electronics, indicated as an addition to my post…..this can get, by one-by-one add-on info, quite complex and many other things keep getting in the way.

From a purely scientific point of view, the concept of electronically measured TDS is quite complex;
and from an instrumentation point of view , that complexity can be hampered by instrument artefacts and approximation assumptions.

In some measurements of water quality, we can confidently and comfortably use approximations on valid assumptions because of the dilute nature of many things that we measure in a fish tank.

With TDS readings, however, the meaning and interpretation is a bit fuzzy when talking about exact quantities.

I prefer to use a conductivity reading and go from there (but I have lost all my conductivity meters in recent house moves).

However, the value of a TDS is more of a quality thing rather than quantity thing.

Look at it like a ‘symptom meter’. Ie it measures ‘symptoms’ (for want of a better word) of what is in the water.

Having a headache is a symptom of something wrong, if the headache gets worse then it indicates that something is worsening…..the headache itself doesn’t exactly tell you what is wrong though. It could be stress, eye-strain, bad gut, or something more serious (and even the headache may cause additional stress to make the headache worse)

Maybe an electronic TDS meter can be likened to “how to tell if someone is rich”.
In that analogy, we could simply go to their bank account and safes and look to see how much money is available and then check if that amount is genuine.

The reality is is that that is not very practical. So we may use other ‘meters’ to scale someone’s money.
Eg looking at their publically overt possessions.
If someone has a new Rolls Royce or a million pound diamond necklace then we may assume that they have money…..and as an approximation, we’d probably be correct.
However, not all rich people spend their money on overt luxuries, and some people may have stolen or hired such luxuries for the day.

So our ‘measure’ is not 100% perfect, but it does tell us something. AND the usefulness of such measures is based upon useful rules of thumb or probabilities. AND based upon our observation of overt possessions, we may even be able to approximate to what of other possessions they might own.
Ie there are TRENDS.

In natural waters, we tend to find trends. It is the realisation of trends that make TDS measurements useful even if not exact.

If I take some pure water and add a load of glucose sugar, then the TDS will be high….but the conductivity would not be anywhere as high as if I had added loads of sodium chloride. Hence, an electronic TDS meter will (because it measures conductivity) show a lower than real TDS for the glucose solution.

But the TRENDS in natural waters allow us to make TDS a useful measure of those trends.

ACCURACY VS PRECISION…..
There is often some debate or confusion over these.
But, if we use a TDS meter as a MONITOR of trends rather than an absolute accurate reflection of quantities then we are on the right track.

Hence, using a single TDS meter and getting to know it is more important than hunting for the most accurate device or cross-comparing readings from different TDS meters. We are in the realms of using the unit as a precision instrument.

If we see a rise in conductivity measured by a TDS meter, then it would be reasonably practicable to assume that the total dissolved solids are increasing…..and that for freshwater natural waters we may even assume that the relationship is somewhat linear for any one type of water (it is not linear in relaity…but if we start looking at non-linear characteristics then we are left with nothing to measure and then have to venture to part 3 of the science !!)

It may also be argued that some Dissolved Solids that are not measured via the TDS meter have varying impacts upon the fish, ie the existence of the dissolved solid might be somewhat inert or negligible in comparison to the Dissolved Solids that do contribute vastly to the conductivity.

TDS vs OTHER MEASURED PARAMETERS.

This is too big a subject to go into too much detail here.

Any compound that is part of the Total Dissolved Solids will have varying effects on other water parameters and on the fish irrespective of whether detected by a TDS meter or not.

Eg citric acid will affect conductivity, TDS and pH (and other things such as RedOx potential). Its addition will affect the conductivity and RedOx and solubility of other compounds (just as a few examples). Thus there is a potential to getting into a cyclic process.

TDS, as with any parameter, is just one part of a bigger picture.
No one water parameter should be the focus of attention.

BALANCING TDS.

Well…what this means really is to realise that when attempting to balance parameters such as RedOx or pH or pH buffering or even softening by use of reactive chemicals, then the TDS and Conductivity could be continually increased on every attempt to, say, balance pH.

This is vitally important…..and it gets to a stage where the increasing of TDS and conductivity will have more of an effect on fish than the actual pH that is being attempted to be corrected.

CAN YOU INCREASE OR DECREASE TDS?

Increasing TDS is easy…..simply add some stuff to the tank such as magnesium sulphate, sodium chloride, bicarb of soda, citric acid, phosphoric acid etc etc etc.
(not that you would actually want to add all of that stuff….but it is just an indicator of how easy to raise TDS).

Reducing TDS is not quite so easy in the fish tank itself.

Boiling the water will reduce the TDS due to components causing carbonate hardness…but you can’t do that in the tank !!! (although your heater may be doing that when you see loads of limescale caked onto it)

Adding certain agents may cause a reaction to reduce overall dissolved solids. Chelating agents may work….some clays even….that can remove dissolved solids.
But do you really want to go down that unknown route?

Passing the water over cation and anion exchange resins (complete de-ionisers) will reduce TDS if due to charged to charged ions (eg salts).
That needs care though.

Domestic Water softeners may be of a single ion-exchange resin….these are not the same as an de-ioniser as they simply exchange ions causing water general hardness with ions that do not cause general water hardness (eg exchange calcium for sodium)

Water changes with part R/O or de-ionised water will reduce the TDS in the tank.
That is the best method (and yet another bit of the reason why water changes are so important).

BODIES OF WATER TDS.

For some bodies of water, we can put a recommended value on the conductivity of the water and even be able to calculate what the TDS should be from known scientific analysis of the water’s contents.

Lake Tanganyika, Malawi, AND the Sea have approximate water qualities in a nutshell.

Building a picture of water quality for those lakes and the sea is almost like painting by numbers.

But when it comes to a general picture for other freshwater bodies, then we can no longer simply paint by numbers……our picture is lot more variable, complex and, is thus, very difficult to place a global recommended TDS reading unless making specific reference to exact locality and time of year (eg the rainy seasons will affect TDS etc).

For conductivity readings from a conductivity meter, it would be relatively easy to give an approximate recommended value for many different types of fish and lakes etc.

To give a reading of recommended TDS would require specific knowledge of the TDS meters conversion factor. Eg a TDS meter that has been calibrated for Lake Tanganyika might give a totally different reading if used in a marine tank (for reasons outlined in the first post).

Remembering that electronic TDS meter readings are really a symptom of many factors then a list of conductivities might be the best way to look at each body of water.

If you have a TDS meter, then knowing the conversion factor, you could convert the TDS reading to conductivity to get a rough guide.

Example….if your TDS meter has a conversion factor of 0.5 (ie calibrated for sodium chloride) then a reading of 100ppm is roughly a conductivity of 100/0.5 = 200 uS/cm.

if your TDS meter has a conversion factor of 0.67 (ie calibrated for general freshwater using the 442 system) then a reading of 100ppm is roughly a conductivity of 100/0.67 = 150 uS/cm.

Some very fishy examples….based upon recommended conductivity.

chocolate gourami, wild heckel discus, altum angels etc
conductivity: 25 to 50 uS/cm
TDS (using 0.67 factor) : 16.75 to 33.5 ppm
TDS (using 0.5): less than 25 ppm

rams, honey gourami, cardinal tetras, discus etc
conductivity: 50 to 100 uS/cm
TDS (using 0.67 factor) : 33.5 to 67 ppm
TDS (using 0.5): 25 to 50 ppm

Angel fish, neon tetras, siamese fighters (not wild ones though), zebra danios (ie our good old common community fish and fish recommended for starters)
conductivity: 100 to 200 uS/cm
TDS (using 0.67 factor) : 67 to 134 ppm
TDS (using 0.5): 50 to 100 ppm

Malawi, most CA cichlasoma-types, platys
Conductivity: 200 to 500 uS/cm
TDS (using 0.67 factor) : 134 to 335 ppm
TDS (using 0.5): 100 to 250 ppm

More specifically for Lake Malawi (being a mid-level Class I rift valley lake), the conductivity is around the lower side of that group…..ie 200uS/cm.
So the TDS using a 0.67 factor instrument would be about 135ppm.

Ie Lake Malawi should not actually have too high a total concentration of dissolved solids.

For Lake Victoria, though, that should not be as high as Lake Malawi even, and would be akin to the waters of a standard community tank of conductivity around 150 uS./cm = 100 ppm TDS.

Tanganyikans, Goldfish, Molies, Scats (brackish), Jordonella floridae
Conductivity: over 500 uS/cm
TDS (using 0.67 factor) : over 335 ppm
TDS (using 0.5 factor): over 250 ppm

Now, with Lake Tanganyika ( a class II rift valley lake) being so massive there is a noted variation from depth to depth and from region to region.
As an average, conductivity between 600 and 730 uS/cm would be recommended (averaging 650)
That relates to a TDS of 400 and 490 ppm (using a 0.67 conversion factor TDS meter). That would mean an average of about 450 ppm

HOWEVER, tank bred Tanganyikans can probably get away with much lower TDS.

My own experiences are mainly with wild caught Tanganyikan cichlids back in the 70s and 80s.
I bought my first lot of tank bred stock about 10 years, and found that they could exist in a ‘malawi’ type set-up; but they did not thrive like the wild caught specimens (deep water species and some of the giants such as Boulengerochromis as well) kept in proper Tanganyikan water.

In proper practice, when quoting or measuring any parameter, the temperature of the water during the test is of great importance. (see my graphs on ammonia testing).
But here, let us assume that the temperature is within the normal tank temperature.

Now, that may have confused more. It gets more and more difficult to make the more complex things easy to understand without reverting to science…..remember, maths and science were invented (well…maybe) to make things easy to explain.

I’ve put this together during coffee and lunch break…..so if I’ve made any calculation errors then please let me know. My memory is not what it is (I remember the conductivities….and then have to calculate the expected TDS meter reading).

ian