Shrimp Tank Water Calculator
Target GH, KH, TDS and pH for your species, and what a water change will actually do to them.
https://calculators.nirajiitr.com/hobbies/shrimp-tank-calculator
How it works
A water change is a weighted average, not a reset. Swapping a fifth of the tank moves the parameters a fifth of the way toward whatever you are adding — which is why correcting a badly-off tank takes several changes rather than one big one, and why that is the right way round. Shrimp tolerate the wrong parameters held steady far better than they tolerate the right parameters arrived at suddenly, because osmotic shock during a moult is what actually kills them.
new TDS = current × (1 − f) + replacement × f, f = change % ÷ 100- f
- Fraction of the water replaced
- TDS
- Total dissolved solids, ppm — the quickest proxy for mineral content
- GH
- General hardness: calcium and magnesium, what shrimp build shells from
- KH
- Carbonate hardness, which buffers pH against swings
Worked example
A 40 litre Neocaridina tank that has drifted to 320 ppm TDS, doing a 20% change with water remineralised to the 200 ppm target.
- 1Water to swap: 40 × 20% = 8 litres
- 2Fraction replaced: f = 0.2
- 3New TDS: 320 × 0.8 + 200 × 0.2 = 256 + 40
- 4Result: 296 ppm
296 ppm — only 24 ppm of movement. Getting from 320 to 200 takes roughly four or five weekly changes, which is exactly the pace you want.
Frequently asked questions
What TDS should my shrimp tank be?
It depends entirely on the genus. Neocaridina, which includes cherry shrimp, want harder water at roughly 150 to 250 ppm with a GH of 6 to 8. Caridina, the crystal and bee shrimp, want much softer water at 100 to 150 ppm, a GH of 4 to 6 and essentially no KH, because they need an acidic pH that carbonates would buffer away. Sulawesi shrimp are the outlier, needing hard alkaline water above pH 7.5. Keeping the two common groups in one tank is a compromise that suits neither.
Why do my shrimp die after a water change?
Almost always osmotic shock, and almost always from changing too much too fast. Shrimp regulate the mineral content of their bodies against the water around them, and a sudden shift forces them to do that faster than they can — which is why deaths tend to appear a day or two later, at the next moult, as a failed shell split. Small frequent changes with parameter-matched water, dripped in rather than poured, prevent nearly all of it.
Do I need RO water and remineraliser?
For Caridina, essentially yes — tap water almost everywhere has too much KH to reach the acidic pH they need, and you cannot reliably remove it. For Neocaridina it depends on your tap: if it comes out around 150 to 250 ppm with a reasonable GH, plain dechlorinated tap water works fine and many people keep them that way for years. Starting from RO and adding a known remineraliser mainly buys consistency, which matters more than any particular number.
How many shrimp can I keep?
The rough guide of ten adults per US gallon, which this calculator uses, is conservative and sustainable for a planted, filtered tank. Shrimp have a very light bioload compared to fish, and established colonies routinely run well past it. The practical limits are food and surface area for biofilm rather than filtration, and a colony that is overstocked tends to simply stop breeding rather than crash.
Is GH or TDS the more useful number?
GH, but TDS is far easier to measure. A TDS pen costs very little and gives an instant reading, whereas GH needs a titration test kit. The catch is that TDS measures everything dissolved, including nitrates and any fertiliser, so two tanks at the same TDS can have quite different GH. Use TDS to track day-to-day drift, and check GH occasionally to make sure the relationship still holds.