GH, KH, TDS and pH without myths
Evidence review: August 12, 2026
These four measurements describe different parts of the water. None of them can tell you whether a tank is cycled, identify every dissolved substance or prove that a shrimp death was a failed molt.
Our recipe, not a species limit: our working recipe is GH 6-8, KH 2-4, TDS 180-250, pH 7.0-7.6 and 72-75°F. Publishing the recipe helps a buyer compare our water with theirs. It does not mean one point outside it is unsafe.
What each test actually measures
| Test | Useful meaning | What it cannot tell you |
|---|---|---|
| GH | General hardness, driven mainly by dissolved calcium and magnesium in ordinary freshwater.1 | The calcium-to-magnesium ratio, or whether other ions are safe |
| KH | A hobby approximation of carbonate alkalinity, the water's capacity to neutralise added acid.2 | The pH by itself, or everything contributing to alkalinity |
| pH | The acidity of the water at the time and place it was sampled | How strongly that pH is buffered, or why it has that value |
| TDS | On a pocket pen, usually an estimate calculated from electrical conductivity rather than a direct inventory of dissolved solids. 3 | Which ions are present, their proportions or their toxicity |
GH and molting: important, but not a diagnosis
Crustaceans move calcium into and out of the exoskeleton through the molt cycle, and environmental calcium can contribute to that balance.4 That supports measuring mineral content. It does not support diagnosing every incomplete molt as "low GH" from appearance alone.
Use the dedicated molting and mineral guide.
We found no controlled N. davidi experiment that compares GH bands and establishes a species-wide optimum or a fatal cutoff. One successful laboratory culture used hardness of 80 mg/L as CaCO3, roughly 4.5 German degrees, while other studies have maintained research colonies in harder water.5 Those are examples of working conditions, not a dose-response trial.
KH and pH are related, not interchangeable
More alkalinity generally means more capacity to resist an added acid. It does not set one automatic pH. Carbon dioxide, gas exchange, photosynthesis, substrate, wood, leaves and the source water all participate in the result. "Set KH and pH follows" is too simple to be dependable advice.
A biofilm culture experiment reported successful reproduction and hatching while pH varied from 8.2 to 9.3 during its shrimp phase.6 That does not make pH 9.3 a recommendation. The experiment tested substrates, not pH. It does demonstrate why a narrow care-sheet range should not be presented as a proven biological boundary.
A TDS pen is a trend tool, not a chemistry lab
Conductivity changes with the amount and identity of dissolved ions and with temperature. A TDS mode converts conductivity using a factor selected by the meter. One NIST meter allows factors from 0.30 to 1.00 and defaults to 0.65. 3 Two meters can therefore show different TDS values for the same sample, and two waters with the same displayed TDS can contain different ions.
Our practice Use TDS to compare like with like:
- Use the same meter and mode.
- Compare the tank with replacement water made from the same recipe.
- Log the trend over time rather than treating one reading as a diagnosis.
- Measure GH separately. Never calculate GH from TDS.
What wild populations do and do not prove
A 2024 field study collected feral N. davidi across pH 6.9-8.6 and conductivity 551-2274 µS/cm.7 That shows the species has established populations across varied waters. It does not prove that every value in that range is equally good for an ornamental colony, or that moving a tank rapidly between them is safe.
An earlier Israeli survey reported Neocaridina across sites with conductivity from 341 to 3150 µS/cm and temperatures reaching 31 C, then maintained one lineage at 20 to 25 C for laboratory observations.8 The authors also retained uncertainty about the exact taxonomic placement of those animals. Wide occurrence and one culture protocol do not identify an optimum, prove equal performance across the range or validate a rapid transfer between waters.
Top-offs and water changes are different jobs
Evaporation removes water while leaving most dissolved minerals behind. A top-off replaces that missing water, so we use RO or distilled water for top-offs. A water change removes water and minerals together, so its replacement water is prepared to the colony's established recipe.
RO is not automatically better aquarium water. It is a low-mineral starting point that provides control. If it supplies the whole water change, remineralise it to the chosen recipe before it reaches the tank. Use the water-change evidence guide and dilution model to keep top-off, export and replacement-water claims separate.
A practical testing order
- If animals are distressed or dying, test temperature, ammonia and nitrite first. GH, KH and TDS cannot tell you whether the biofilter is working.
- Test the tank and the prepared replacement water with the same tools.
- Record GH, KH, pH and TDS with the date and meter mode.
- If the established colony is reproducing and growing, treat its recorded water as the baseline. Do not correct it merely to match an internet chart.
- If a change is necessary, alter one input and measure again. Research does not establish a universal safe rate of GH or TDS change for N. davidi.
Use the reproducible water-testing record.
Sources and measurement references
- U.S. Geological Survey. Hardness of Water. Definition and reporting of calcium and magnesium hardness.
- U.S. Geological Survey. Alkalinity and Water. Acid-neutralising capacity and pH buffering.
- National Institute of Standards and Technology. Conductivity meter manual. Conductivity, temperature compensation and TDS conversion factors.
- Greenaway, 1985. Calcium balance and moulting in the Crustacea. Biological Reviews 60(3): 425-454.
- Tropea, Stumpf and López Greco, 2015. Effect of Temperature on Biochemical Composition, Growth and Reproduction of the Ornamental Red Cherry Shrimp. PLOS ONE 10(3): e0119468.
- Viau et al., 2016. Assessment of a biofilm-based culture system within zero water exchange. Aquaculture Research 47(8): 2528-2542.
- Maciaszek et al., 2024. Invisible invaders: range expansion of feral Neocaridina davidi. Biological Invasions 26: 2407-2424.
- Levitt-Barmats et al., 2019. Life-history traits and ecological characteristics of the ornamental shrimp Neocaridina denticulata, recently introduced into the freshwater systems of Israel. Aquatic Invasions 14(4): 684-702.