A TDS display is not a general-hardness test
A conductivity-based TDS meter does not directly measure aquarium GH. It measures an electrical property and converts the reading into an estimated dissolved-solids display using a selected factor. GH concerns hardness, principally associated with calcium and magnesium, and requires an appropriate method for that parameter. The two results can both be useful, but their similar ppm labels do not make them interchangeable.
This educational guide explains measurement identity, not suitable conditions for fish or a remineralizer dose. Our Water Hardness Converter changes the reporting scale of an already identified GH or KH/alkalinity reading. It has no TDS-to-GH mode. Entering a TDS number in its ppm field would not cause the calculator to discover the missing calcium and magnesium information.
Identify what the meter actually reports
Hanna explains that conductivity meters can display estimated TDS through conversion factors, including commonly used 0.5 and 0.7 conventions. The instrument's factor and temperature-compensation basis belong with the reading. A number recorded only as 200 ppm can be ambiguous if the reader does not know whether it came from a hardness test, an estimated TDS display or another measurement altogether.
TDS itself is a broader dissolved-solids quantity than hardness; a meter's conductivity-derived display is an estimate of that broader quantity, not a selective calcium-magnesium assay. Do not remove the EC or TDS label from a record because another aquarium guide uses ppm. A unit label without the parameter and method is insufficient to establish what the result can legitimately be converted into.
Work through two factor settings on the same EC
For a hypothetical conductivity reading of 400 µS/cm, factor 0.5 yields a displayed estimate of 200 ppm, while factor 0.7 yields 280 ppm. The 80 ppm difference comes entirely from the chosen reporting factor. It does not represent an actual addition of dissolved material or a measured change in GH between the two displays.
These numbers illustrate the factor arithmetic, not an endorsement that either conversion exactly matches every sample's dissolved solids. Always check the actual meter instructions rather than assuming the factor from its appearance or country. If comparing two instruments, record the underlying conductivity where available and the factor used. Do not average unlike ppm displays and call the average a more accurate hardness value.
More than hardness ions can affect the signal
A water sample can contain dissolved ionic constituents other than those represented by a general-hardness test. A conductivity measurement responds to the sample's electrical behavior as a whole, not a label identifying which ions contributed. Therefore one EC or derived TDS value does not uniquely determine the calcium and magnesium information needed for GH. Additional chemical information is missing, not merely a numerical conversion factor.
This is why the problem cannot be fixed by learning a single ratio from one tap-water sample and applying it everywhere. A local correlation under stable conditions is not a universal chemical identity. Source blends, additives or changes in composition can alter that relationship. This article does not propose a substitute calibration that claims to turn an ordinary TDS tester into a validated selective hardness instrument.
Do not feed TDS ppm into a CaCO3-equivalent conversion
The converter uses 17.86 mg/L as CaCO3 per degree for the same selected parameter. Dividing a hypothetical 200 TDS ppm display by 17.86 gives approximately 11.20 as arithmetic, but it is not a measured 11.20 dGH. The missing premise is that the original 200 described hardness on a CaCO3-equivalent basis, which a TDS display does not establish.
That same number also cannot be relabeled KH or pH. Unit conversion preserves the underlying parameter; it does not identify it. The existing degree-to-ppm guide explains equivalent reporting in more detail. Keep TDS, GH, alkalinity and pH as separately identified observations rather than using a converter's valid mathematical response as evidence that an incompatible input was scientifically appropriate.
Use matched records to answer narrower questions
A hypothetical log can record a TDS estimate of 200 ppm with factor 0.5 and an independent GH result of 6 dGH. The latter converts to 107.16 mg/L as CaCO3 under our tool's convention. The difference between 200 and 107.16 is not automatically a measured amount of other minerals; the numbers describe different methods and reporting bases and cannot simply be subtracted as physical components.
The independent GH test answers the hardness question that the meter alone could not answer. Repeating those paired observations may help describe a particular source, but it does not establish a universally transferable ratio or a reason to stop relevant tests. Record date, source and method together. A stable meter display is not proof that all water properties or animal-care conditions are unchanged.
Do not convert a useful observation into a treatment target
An unexpected conductivity-derived TDS reading does not specify which product to add, which parameter changed or how much water to replace. A display does not choose a suitable GH target for livestock. Likewise, a low TDS value does not establish that purified or mixed source water is ready for an aquarium. The relevant measured parameters and husbandry context still need to be established.
This guide gives no universal TDS target, safe maximum or additive formula. Follow appropriate product and testing instructions rather than correcting one display until it resembles an online example. If results are inconsistent, check the method, units, calibration and sample context before assigning a chemical cause. A mathematical mismatch between TDS and converted GH is not itself a diagnosis of a water problem.
Choose the converter only after identifying the parameter
Use the Hardness Converter when the original result is explicitly GH or KH/alkalinity in supported degree or CaCO3-equivalent units. Retain the original test result, method and realistic precision. Use the test-reading article for manufacturer-specific drop-count conventions, and the GH-versus-KH article for why two hardness-related labels still represent distinct properties despite sharing reporting scales.
For a conductivity meter, preserve EC, displayed TDS, factor and temperature basis as applicable. The responsible answer to whether it measures GH is no, not by a universal conversion of its ordinary TDS display. A useful tool remains useful within its scope; claiming that one inexpensive number answers every water-chemistry question would obscure the very information needed to interpret it correctly.