GH, KH and TDS in a shrimp tank: what each one actually measures

Quick answer

GH, KH and TDS are three different measurements that shrimp keepers routinely treat as one. They are not interchangeable, and knowing which one does what is the difference between a colony that breeds and one that slowly fails.

GH is the mineral content shrimp build their shells from. KH is the buffer that stops pH moving. TDS is a summary reading of everything dissolved, useful for spotting change but incapable of telling you what changed.

Shrimp tolerate a surprisingly wide range of water. What they do not tolerate is that water changing quickly.

• GH is calcium and magnesium. It is the number that governs moulting.
• KH is carbonate buffering. It is what holds pH steady, and pH problems are almost always KH problems.
• TDS measures conductivity and converts it to a figure. It cannot distinguish minerals from waste.
• Neocaridina and Caridina shrimp want different water. A single set of targets does not cover both.
• Stability beats any target. A slow move to a slightly wrong figure is safer than a fast correction to the right one.

Decide in 60 seconds

GH: the number that governs moulting

General hardness measures dissolved calcium and magnesium.

A shrimp grows by shedding its exoskeleton and building a new one, repeatedly, throughout its life. The material for that shell comes from the water and from the food, and the water is the larger contributor.

Too little GH and the shrimp cannot harden a new shell properly. Too much and the old shell becomes difficult to shed. Both show up as the same visible outcome: an animal that dies part-way out of a moult, or one that develops a visible gap at the join between the head shell and the body and cannot complete the separation.

GH is therefore the first number to look at when moults go wrong, and it is the number a remineralising product is designed to set.

Target values differ by species group and are worth getting from a specialist source or from the breeder you bought from rather than from a general article. What is universal is that the value should be stable, and that it should be set deliberately rather than inherited from whatever comes out of the tap this month.

KH: the buffer, not the pH

Carbonate hardness measures carbonates and bicarbonates. Its job is to absorb the acids that a tank continuously produces and stop pH moving.

The relationship is easy to misread. KH does not set pH. It resists changes to it. A tank with meaningful KH holds a steady pH; a tank with almost none can swing sharply from small causes, and swings are what kill shrimp.

This is where the two main shrimp groups diverge most clearly.

Neocaridina shrimp, the cherry shrimp and their colour variants, are generally kept in water with some carbonate buffering and a stable neutral to slightly alkaline pH. A remineralising product that raises both GH and KH is the usual choice.

Caridina shrimp, the crystal, bee and Taiwan bee types, are generally kept in soft, acidic water with essentially no carbonate buffering, achieved with an active soil substrate that actively pulls pH down. A remineralising product that adds only GH is the usual choice, because adding KH works directly against the substrate.

Using the wrong product for the group is one of the most common and most expensive mistakes in the hobby. Check which one you have before you buy anything.

Active soil substrates are also consumable. Their buffering capacity exhausts over months and years, and a Caridina tank that gradually stops holding its pH is usually a substrate reaching the end of its working life rather than a new problem.

Mixing and testing the replacement water before it goes anywhere near the tank is the whole technique.

TDS: useful, and routinely misunderstood

A TDS pen passes a current through the water, measures conductivity, and converts that to a figure in parts per million.

It is fast, cheap and genuinely useful. It is also blind to composition. The reading counts the calcium and magnesium you added on purpose, the carbonates, and also the nitrate, sodium, phosphate and dissolved organic waste you did not.

That produces two practical consequences.

A TDS reading cannot replace GH and KH tests.

Two tanks reading identically can have entirely different mineral profiles. If your GH is drifting down while nitrate climbs, TDS can stay flat while the water becomes unsuitable.

A rising TDS between water changes is usually waste, not minerals.

Nitrate and dissolved organics accumulate as the tank runs. Watching that number climb and then fall after a water change is a good, quick indicator that the maintenance routine is working.

Evaporation is the exception that catches people out. Water leaves; minerals do not. Topping up with remineralised water adds more minerals on top of the ones already concentrated, and TDS climbs steadily. Top up evaporation with pure water, reverse osmosis or distilled, and use remineralised water only for actual water changes where water was removed.

Calibrate the pen occasionally, and take the reading at a consistent temperature, because conductivity varies with it.

Measure the new water and the tank water before the two ever meet. That single habit prevents most shrimp losses.

Why stability beats the target

Shrimp colonies survive across a surprisingly wide band of water chemistry provided it does not move. What kills them is the rate of change.

A shrimp regulates the salts in its body across a permeable surface. A rapid change in the surrounding water forces a rapid internal adjustment, and that is stressful at best and lethal at worst. It also frequently triggers a moult, which is the single most vulnerable moment in a shrimp's life, at the exact point where the water is unfamiliar.

That gives three working rules.

Change small amounts, often, rather than large amounts occasionally.

Match the replacement water to the tank before it goes in.

Mix and test it in a separate container. Temperature, GH, KH and TDS should all read close to the tank's values.

Add it slowly.

Dripping or pouring in gently over a period, rather than tipping it in, keeps the transition gradual.

The same principle governs acclimating new shrimp. Drip acclimation over a long period is the standard for a reason: shop water and your water are rarely close, and a shrimp moved between them quickly may look fine for a day and die on its next moult.

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Do not correct a wrong parameter quickly.

If you discover that GH, KH or TDS is well outside where it should be, the instinct is to fix it today. That correction is a larger and faster change than the original error, and it is what kills the colony. Move the tank toward the target over a series of small water changes across days or weeks. Shrimp survive being in slightly wrong water for a while. They frequently do not survive being rescued from it.
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The testing routine

What never to do

Do not use a GH and KH remineraliser in a Caridina tank with an active buffering soil. The two are working against each other and the substrate will exhaust faster.

Do not use pH up or pH down products. They move the reading without changing the buffering, which produces exactly the swing you are trying to avoid.

Do not top up evaporation with remineralised water. That is how TDS climbs steadily in a tank nobody has touched.

Do not do a large water change to fix a bad reading. Make the correction gradual.

Do not add new shrimp without drip acclimation, however similar the water looks.

Do not rely on a TDS pen alone. It tells you that something changed, never what.

Do not assume tap water is consistent. Supplies vary seasonally and by works on the network, and a tank that has been stable for a year can be disrupted by water that arrived different one week.

When to get help

Shrimp are seldom seen by a general veterinary practice, so the people to ask are an aquatic veterinarian where available, an experienced dwarf shrimp keeper, or a specialist retailer.

Ask quickly if you have a run of failed moults, deaths clustered in the days after a water change, or a colony that has stopped breeding without any obvious cause.

Bring the numbers, not the impression. GH, KH, TDS, nitrate, temperature, your source water, your substrate type and your remineralising product answer nearly every question that gets asked.

Steady readings, clean moults and a colony that quietly grows are what correct water chemistry looks like from the outside.