Hi
@Mattant1984, I will only add a few points to
@KirstyF's excellent comments above:
A TDS meter determines the amount of dissolved solids (minerals) in a body of water that carries a charge such as calcium, magnesium, potassium sulfates, nitrates, chlorides among others. Keep in mind that the measurement is not telling you anything about which minerals makes up the TDS - it could be all calcium . It's just a number.
The TDS we measure is actually electrictical conductivity - often express as Micro-Siemens per Centimeter or uS/cm. In purified water that is stripped from all these minerals (i.e. distilled water) you will measure a TDS close to zero - in other words, the water won't conduct any electricity because there are no ions to facilitate the process.
Lots of TDS meters, although they all internally measure uS/cm, only displays TDS ppm by internally converting the uS/cm to TDS. There are a lot of
good reasons for that; by far most of these meters sold, are used to measuring pools, spas, household water, household water purification systems, DYI herbal beverages etc. etc. and almost all the literature you will find on these topics refer to TDS. Unfortunately, conversion from uS/cm to TDS vary quite a bit among meters - the conversion is just a simple scalar (its usually 0.5, 0.55, 0.64 or 0.7 - depending on region and meter range) - so for instance if you measure 100 ppm with a TDS meter that uses 0.5 your actually measuring 200 uS/cm ( TDS = 200 uS/cm * 0.5) or conversely if your water is known to be 200 uS/cm and measure TDS with a meter that uses a conversions factor of 0.7 you meter is going to read 140 ppm (TDS = 200 uS/cm * 0.7) - quite a difference from exactly the same electric conductivity! Unfortunately the hobby has widely adopted the TDS nomenclature which makes it hard to compare say a TDS value from one hobbyist to another or comparing something you read in an article or a forum post etc. without knowing what the reference (conversion factor) is. All worth keeping in mind when comparing your reading to others or when taking advice on specific TDS values.
Now, I consider a TDS meter invaluable; to me it's close to be as important as a thermometer. The simplest use is just to ensure you don't have large fluctuations in TDS over time. Whether you want to target any specific values is up to you and
mostly a livestock consideration (more on that later), but what you don't want to see is a large unanticipated drift over time, but rather a consistent and stable reading say before each water change. As mentioned, TDS is just a number... it says nothing about what the number is made up of. However, it gives you a hint about change, be it anticipated or unanticipated - if its unanticipated you will have to figure out the reason and source (waste buildup, leaching substrate, leaching hardscape, varying water source parameters, accidental over/under dosing of minerals or fertilizers etc.). If your tank is mature, your water source is consistent and you are maintaining your tank consistently and dosing consistently you will see a consistent TDS reading within a reasonable margin over time - say before each weekly water change - whatever that number is. You will often hear that TDS wont measure say organic waste from fish, food and plants. That is mostly true, but as the organics break down it will show up contributing to TDS - after all, most of those waste products are ultimately made up from the minerals mentioned above.
As a rather extreme example (for me at least), I was unable to maintain my tanks for some period of time in the late spring and early summer where I had to go 5 weeks without a water change (the fish went on a lean diet and plants essentially got no fertilizer) and that really showed up as a big spike on TDS - and not quite as much as the state of the tanks suggested just from a visual inspection, although some plants started to look lackluster and tanks had some visible dirt buildup - so there is no doubt in my mind that TDS a terrific companion to our eyes... it's just a plain good aid to make sure nothing is out of whack, as
@KirstyF puts it! … before we realize so with fish, plant and algae issues inevitably following closely behind…
Besides working as a
litmus test for consistency and swings that may merit further investigation as described above, I am rather
hawkish in keeping TDS low for the sake of my livestock. Keeping TDS as low as possible while maintaining excellent plant health has become somewhat of an
obsession of mine
It mostly comes down to osmotic regulation, which at elevated TDS levels requires a great deal of energy from our livestock, and at higher levels causes stress and affect their wellbeing and life expectancy - and this may not be obvious until the fish or shrimps just dies for no apparent reason - sort of like hypertension in humans. I dont think you have to be a scientist to acknowledge that trying to mimic the natural habitats of our livestock in terms of critical water parameters they have adapted to over eons of time is anything but positive and keeping TDS low is one of the tools that I use to maintain that goal.
An ideal range is much harder to determine and advice about. Most soft water fish originated or originates (if wild caught) from waters that are in the 10-100 ppm range (20-200 uS/cm) mostly in the lower part of that range. If you wish to run a lush planted tank you will find it very difficult to get
much below say 50 ppm due to the necessary amount of nutrients that you will have to provide for the plants on a regular basis and that’s even if you’re in full control of your water (using Reverse osmosis water only and carefully remineralize and dose on top of that). Furthermore some livestock such as shrimp have the audacity to require both a certain (relatively high) amount of calcium and magnesium and low TDS at the same time which makes it even harder if you also have a lot of plants to provide for as well. My own densely planted shrimp tanks that also hosts tetras hovers around 100 ppm (200 uS/cm) - it took quite a bit of effort to get there and I’m not entirely happy about the situation yet
It obviously would have been impossible without being able to monitor the TDS
Anyway, that turned out to be just about everything of value I know on the topic
Short answer: yes, get a good TDS meter that shows both ppm and uS/cm and use it as a sanity test for the state of your tank!
Cheers,
Michael