What Is Totally Dissolved Solids ? (TDS)

If ever there was a subject full of mystery misunderstanding and the cause of many an argument it is TDS.

One article will not stop that. Indeed, the following paragraphs will no doubt have many who disagree with its’ content. Such is the nature of free speech however, that I get to say what I think about it and you may take it as you will. I have however attempted to simplify things and in layman’s terms to appeal to the widest audience. For those with a scientific background please bear this in mind.

What is TDS?

Total Dissolved Solids in the context of aquaria refers to the total amount of dissolved solids within the water we may utilize for our tanks. This may be in our taps, well water, RO water (remineralised or otherwise) and in our tanks. These solids may be in the form of calcium, salts and other minerals, metals, contaminants, chemicals, and pretty much anything that will dissolve. These solids may be naturally occurring, added to our tap water by water companies, enter by rainfall, Run-off from fields, industrial pollution or deliberately added by ourselves when adjusting the water we have. Therefore, TDS is used as one of the parameters for our fish and shrimp-keeping.


And so, we come to one of the biggest and misused/misunderstood issues in measuring TDS. Knowing how high or low our TDS measures will tell us only the amount of dissolved solids. There is no way from a single number, test or measurement that we can know what those solids are nor how much of each are present, unless you started with water with a reading of zero solids and added a single known substance to it, or a mixed substance with clearly defined ratios of components. I hope one thing that is now clear is that TDS is not, as is commonly considered, a measurement of Hardness. It is conceivable to have soft water with high TDS simply because of the nature of those solids. Not all solids make water harder.

How do I measure it?

The most common means of testing in the home aquaria is be means of a hand-held device, commonly referred to as a “TDS Pen” These range from under £5 to well over £300. Some of them can be calibrated to account for inaccuracies over time, whilst the cheaper models frequently cannot.

An accurate way to measure TDS would be to take a measured amount of water, boil off the water and measures the solids left behind in the boiling vessel after all the water has evaporated. Hardly the most practical option at home. So, we use a “pen”.

Typically, a TDS pen doesn’t actually measure TDS. “huh?” I hear you cry!

So how do they work? 2 electrical probes very close together (but not touching) at the end of the pen are submerged in your water. One sends electricity, and the other receives it. As you increase the dissolved solids in the water, we can expect the receiving probe to receive more electricity. This is because dissolved solids increase Electrical Conductivity. The TDS Pen will measure the received electricity and extrapolate, from a “comparison standard”, the expected percentage of solids dissolved in water that that is likely to result in the measured amount of received electricity. This extrapolated figure is then represented, normally in “parts per million” (PPM), in the TDS pen display. It is not 100% accurate as different dissolved solids affect electrical conductivity differently, but it’s about as accurate as we can get at home. Any minor discrepancy from the true figure can be ignored for our aquatic purposes.

Please note all TDS pretty much need to be regularly calibrated. A link to some calibration fluid for this purpose can be found at the end of this article among our other products. Click on the link and look for “TDS Calibration fluid”.

Great! I know my TDS, now what?

This will depend on the source of the water you tested, what you know about the content, and what you are aiming to do with it.

As previously explained, if you are testing a water source of unknown content, (Mains tap water, well water, local river, and potentially even your tank etc.) the result of testing is of limited value. You have a total number but without knowing what it is made of, there is limited value in it. external sources such as your tap, well or river water may have all manner of dissolved solids and you have nothing more than the total number of them expressed in parts per million. If I said I had 5 litres of liquid in a container, without knowing what that liquid was you wouldn’t know whether you could drink it, clean your drains with it or put it in your car’s fuel tank!

There is a little difference with your tank. Here we can at least assume it isn’t caustic soda or Diesel! If we have taken care to put water in either by treating tap water, or by using RO/DI water (See elsewhere for explanations on Reverse Osmosis and De-Ionised water if needed) then we can limit the likely substances within our tanks. In addition to water there may be Tannins from leaves or various wood decorations, chemicals restricted to those found in plant fertilizers and fish/shrimp treatments etc. The latter should be identified at least from the manufacturer, and should be harmless and in some cases, temporary.

This seems a good a time as any to point out that whilst water evaporates, the dissolved solids within it do not. Therefore, over time as evaporation takes place in your tank it is expected that the TDS will rise. This is a good case for lids/evaporation trays, but that is a whole different subject not for this discussion.


So, is the TDS reading ever a really useful thing?

Put simply, “yes”, provided you are fully aware of what you are and perhaps more importantly, are not measuring.

Let’s take our tap water first of all. We measure the TDS and get a reading. Great! Let us suppose it is 34 (I use this figure as it is the reading of my own tap water) So I know there isn’t a great deal of dissolved solids in it. I also know plants fish and shrimp need minerals and salts etc. so I know I am going to need to do something to increase them. The downside is I don’t know how much of that 34 ppm is something nasty like copper which could potentially kill a lot the inhabitants of our tanks. I could ask my local water company but am not sure how much I could rely on their response being accurate! What I do know is that they are required to keep the water at least safe for human consumption in this country.  I know Chlorine/Chloramines are added and they will account for a percentage of the TDS so I can treat for those. Indeed just 2-3 days prior to starting this article my wife Rosh and I both noticed a very strong smell of chlorine in our water. The following day it seemed to have improved and today appears fine. If I therefore remove chlorine/chloramines I can expect the TDS to go down further. I might then remove these, retest the TDS then consider remineralising the water to the GH and KH (General and Carbonate Hardness) readings I want for our tanks and shrimp.

Let’s now consider the tap water if it were to read a TDS of 500. Not quite “liquid rock”, but that’s a lot of dissolved solids. What on earth could they be?! Without further testing we cannot hope to know. One thing we can do is consider where we live. Despite now living in Scotland I am originally from Kent in the far south-east of England. Kent is pretty much built on Chalk, hence the “White cliffs of Dover”, so has a high carbonate hardness with lots of dissolved minerals in the water. This would account, in my opinion, for a great deal of that 500 TDS were tap water to be found with that reading in my hometown. But not all. I would still personally want to know what else was in there. For the majority of my life in Kent I kept fish and used tap water. I never treated for chlorine, favoring instead to aerate water overnight prior to using it in a tank, to allow the chlorine to disperse and vanish into the ether. (ok bit poetic but basically it got out of the water sufficiently by aerating it for 24 hours. Note this does not work for the more recently used Chloramines)


The water by nature of the high carbonates was very hard and as a result I tended to keep more fish better suited to that kind of water (Tanganyikan cichlids in particular)

Shrimp and invertebrates on the other hand are far more sensitive and the KH and GH were well beyond what is generally considered acceptable for those species to thrive. That said it was so many years ago few shrimp were available and certainly they did not have the popularity they now enjoy. Many species had not been discovered or entered the hobby and certainly the color forms now available weren’t even dreamt of.

Finally, what about water with a TDS of lets say, 160? Well most shrimp could manage with it, except…. We still don’t know what it is made of! Providing the TDS is made up of “good stuff” we are good to go! If not, it might be “shrimpocide”! ☹

So, we can measure dissolved “stuff” in our water but not know what it is. Is there really then any point measuring it?

Actually, yes in several instance it can be invaluable, and it is a great and useful tool provided (if I haven’t said this enough already!) you are aware and mindful of its limitations.


Lets look at a few instances where it is may be used…

  1. Stability

Measure your tank water TDS when all is well. Measuring it regularly after this may give an early indication that something is changing and going awry. This may then lead you to a more thorough investigation testing for individual substances as you typically may have in your home test kits like GH, KH, PH, Nitrogen cycle etc. Some of us also go beyond and test for copper, magnesium, calcium, phosphates, iron, silicates and all sorts of stuff! I have already indicated evaporation will increase TDS and you can keep an eye on this with a TDS meter. Yes you could check all those weekly if you wanted to but its an expensive thing to do given the cost of test kits especially with multiple tanks. It can also be time-consuming. A TDS check is quick and whilst not as thorough it may be sufficient to mean you can do the more thorough checks less frequently. It is NOT a replacement for those tests, merely and additional tool to assist.

2. Tap Water consistency

So, you know your tap water TDS as you have checked it. Checking it every time you perform a water change may give you a warning something has changed. In recent weeks due to the COVID-19 pandemic many water companies all around the world have changed their water treatment procedures. They have, in some instances, changed or increased chemical dosing to the point that many people have reported mass die-offs in tanks as they were unaware of the changes and continued as they have always done. It is possible a simple 2 minute TDS check MAY have alerted them and given sufficient cause for concern that they investigated further and possibly saved the many poor creatures that have suffered as a result.

3. Setting up your tank.

Knowing your tap water TDS may help decide what to keep in it. Pick species likely to do well in the water you have or be prepared to do some work to get the water parameters for the species you would like to keep!

4. Diagnosing problems

Shrimp dying? Moulting problems? Strange behaviors? TDS wont give you the answer but it might point you in the direction to do further tests or rule some things out.

5. Remineralising – the easy way

Finally the most use you may find is when remineralising RODI water (or low TDS tap water). This filtered water will have a TDS close to zero. So you take your water and remineralise it to the desired GH and KH readings. Lets assume you are using one of the proprietary brands of remineralising products like Salty Shrimp. Once the desire GH/KH is found then measure your TDS. Next time you remineralise more water you should only have to reach the same TDS and your GH/KH should be correct. This saves test kit solutions every time. Please note when replacing the pot of minerals with a new batch you should perform the check again as they are not always consistent.


How do I change the TDS?

First of all determine if you NEED to. Remember TDS is of limited value. First find your KH and GH readings as these are of far more value. If these are within the accepted parameters for your tank livestock and they are doing well, you have to consider, if it works, why do you want to change things?

Assuming you are ready move on and change things, next consider what do you need to change – KH? GH? Both?

To reduce TDS and KH/GH you can gradually add some RO/DI or distilled water to dilute it. This should be slowly over a period of days/weeks not hours if you have a large change to be made. Of course, if they are currently no livestock go for it much quicker.

To raise the TDS, you might select a remineralising product such as Salty Shrimp GH/KH+ or KH+. This can be added to some water and mixed prior to a water change. These dissolve very fast. (I wouldn’t recommend adding these directly to the tank due to changes being too quick in the tank shocking any livestock, and if you accidentally put in too much it’s hard to quickly remove it!). In a water-containing vessel add some of your chosen product to the new water and measure the KH and GH. When it reaches the desired result, you can then add this to the tank (after first removing some water to make room for it!) I would advise adding it by first warming it to around the same temperature, then dripping it in over a period of time. In a 180 litre tank we typically change 20 litres during a water change. This is around 11-12% water changed. We use 10 litre containers when refilling and add it over the course of an hour knowing the parameters of the new water are very close to those in the tank. For large water changes, and/or with parameters far different in the tank and new water, take longer. You cant really add it too slowly but you sure can go too quick!

There are a number of DIY methods to control the returning flow rate, tying knots in airline etc. but we aren’t fans of those. Having noted the difficulty trying to get the flow right and experiencing large amounts of water on the floor when the airline falls out of the tank or the water change containers, we decided to produce a far more robust and user friendly tool which is available now among our Ebay products.…There is a link at the end of this article to our products where you my find these. Look for the Drip Acclimation sets, they make the whole process so much easier!

Other ways to increase TDS is the addition of crushed coral, used frequently to help with moulting issues with Neocaridina. You will have to monitor this closely as it may increase GH/KH and PH with little control. Other methods and substances are widely reported online. I recommend you talk to others in your forums, ask questions and research to find them and the pros and cons of using them before you dive in.

One subject I have not discussed here is “what TDS do I need?” This has been deliberate. First of all I think I have made it clear that TDS is a tool not a target. If you get your other parameters correct (PH/GH/KH etc) the TDS measurement should be more of quick monitoring as a way of indicate all may not be well, and an aid in the diagnosis of any problems. If you have those parameters correct and your TDS measures way outside the “expected norm” for those parameters or is changing, it is telling you to investigate further. It does NOT diagnose the problem. Rather it may tell you a problem of some description is developing. I refer you to the above section “Lets look at a few instances where it is may be used…”

For more information on the GH/KH/PH (ok and TDS!) required for our livestock there are many resources online and many people claiming to have thriving colonies way beyond those “norms”. Again that is not something for discussion here within this article.



TDS measurements are a useful aid in the aquarium and shrimp-keeping context. Understanding what it is, what it means, and its’ limitations are critical to benefiting from a TDS Measurement and understanding why you should not take remedial action based solely on a TDS reading. We can measure it and we can change it but should only attempt to change what we understand.

I hope this has been of some use and will be happy if you have taken or learnt anything from it. If you don’t agree with it that’s fine too! I am happy to discuss further anytime. Come find us in the link below.


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Del Clarke – Delmarine Aquatics Sept 2020

Categories: Shrimp Tank Chemistry