How much water should you change in a shrimp tank?

Evidence review: August 12, 2026

There is no controlled Neocaridina davidi experiment establishing one water-change percentage or calendar for every aquarium. A useful routine has two parts: a repeatable baseline for one tank, and a measured reason for changing it.

Why the studies do not produce one schedule

A 126-day culture study used twelve 60 L aquaria, continuous aeration, three artificial biofilm substrates and zero water exchange. Evaporation losses were replaced weekly. In its second 63-day phase, shrimp reproduced and had high survival while the designed biofilm system helped process nitrogen compounds.¹ This shows that one engineered zero-exchange system worked. It does not show that an ordinary aquarium can skip water changes, food control or measurement.

A different 90-day density study kept juveniles in 2 L units with continuous aeration, daily formulated food and complete weekly water replacement.² It tested density, not water exchange. The two studies demonstrate how strongly a result travels with the complete system. Neither compared 10, 20 and 50 percent changes or established a home-aquarium optimum.

What a partial change can calculate

For one well-mixed dissolved constituent, immediately after a change:

new = current x (1 - fraction changed) + replacement x fraction changed

If tank nitrate is 40 mg/L, replacement water is 10 mg/L and 25 percent is replaced, the model gives 32.5 mg/L. It does not give 30. If replacement water is higher than the tank, the result rises instead of falling.

Model limits: all values must use the same unit and analyte. The calculation assumes complete mixing, unchanged volume and a constituent that is not produced, consumed, precipitated, adsorbed or released while the changes occur. It does not determine whether a concentration is safe. Conductivity-derived TDS cannot be entered as if it identifies nitrate, ammonia or another particular substance.

Top-off and water change are different operations

Evaporation removes water while concentrating many dissolved constituents in the water that remains.³ A top-off replaces lost volume. It does not export nitrate, dissolved minerals or another retained constituent. A water change removes a fraction of water and adds a measured source batch.

We use RO or distilled water for evaporation top-offs so the top-off does not add another mineral load. That is a house recipe choice, not a treatment for high TDS or an unexplained death. Plants, precipitation, food, fertilizer, substrate and removed material also change the tank's chemistry over time.

Ammonia, nitrite and nitrate are not interchangeable

A 2023 doctoral experiment exposed juvenile N. davidi for 96 hours in a control plus five total-ammonia exposure concentrations and a control plus five nitrite exposure concentrations, with two replicates per treatment. It estimated median lethal concentrations of 19.98 mg/L total ammonia nitrogen, 0.48 mg/L unionized ammonia as nitrogen and 0.69 mg/L nitrite as nitrogen under its test conditions, with greater gill damage at higher concentrations.⁴ These acute endpoints are not recommended aquarium targets, chronic no-effect levels or evidence that a lower detectable reading is harmless.

Total ammonia contains ionized and unionized forms. The more toxic unionized fraction depends strongly on pH and temperature, so a total-ammonia number cannot be interpreted without the test's reporting unit and the water conditions.⁵ These are three nitrogen-basis results: N-TAN, NH3-N and NO2-N. They cannot be compared directly with a kit reporting NH3 or NO2 ion mass. Use the nitrogen reporting guide and translator. Our operating response to total ammonia or nitrite above the chosen methods' detection limits is to verify the result, protect aeration, stop adding the source, prepare known replacement water and investigate the processing failure.

This review found no controlled chronic nitrate-only trial in N. davidi that validates the common 20 mg/L cutoff. We keep nitrate under 20 mg/L on our registered reporting basis as a house action level and follow its trend, but do not publish it as a species toxicity boundary. A nitrate result also says nothing about dissolved organics, fertilizer, metals, chlorine, chloramine or every other reason to exchange water.

Prepare the source before choosing the percentage

1. Identify the source. Read the current utility water-quality report and service notices. Utilities can switch between chlorine and chloramine.
2. Use a treatment that explicitly names the disinfectant. Follow the current product label for the volume being prepared. Do not rely on smell or a generic dose copied from a forum.
3. Do not age chloramine away. CDC guidance says free chlorine may dissipate after water sits for days, but chloramine cannot be removed that way. Both must be removed before water contacts aquatic animals.⁶
4. Measure the replacement batch. Record source, date, temperature, GH, KH, conductivity or TDS and any analyte that caused the change. A utility report is historical context, not today's bucket measurement.
5. Calculate the expected direction. Replacement water cannot dilute a constituent below its own concentration through mixing alone.
6. Refill as a controlled operation. Avoid direct current on the animals, watch for leaks and record the actual volume. No controlled N. davidi evidence establishes one universal refill speed.

Where the nitrogen-processing community lives

Aquarium nitrifiers are not one bottled species floating mainly in old water. Established freshwater biofilters contain attached microbial communities. A study of three aquarium nitrogen balances attributed at least 81 to 86 percent of total nitrogen conversion to the biofilters it tested.⁷ A later survey detected clade A comammox Nitrospira genes in all 38 freshwater aquarium biofilters and found them dominant among ammonia oxidizers in 30.⁸

Removing water is therefore not the same operation as replacing established filter media. This does not mean the water is sterile or every useful organism lives in one sponge. It means "save old water to save the cycle" is the wrong model.

Clean a filter for a measured reason

Clean when flow, air delivery or accumulated solids show that service is needed. Do not replace all biological media on a package calendar, and do not deep-clean the filter, substrate and every conditioned surface in one event.

Our conservative practice is to squeeze sponge media in removed aquarium water or prepared dechlorinated water, preserve part of the established media and restore flow promptly. A drinking-water pilot found chlorinated backwash prevented nitrification from developing while dechlorinated backwash allowed it.⁹ That was not a home-aquarium tap-rinse experiment. This review found no direct test proving that one brief tap rinse instantly sterilizes a mature aquarium sponge. Avoiding an unmeasured disinfectant exposure is prudent; claiming instant total bacterial death is not.

After unusual filter service, record flow and test total ammonia nitrogen and nitrite with the same methods used for the tank baseline. The observation that the filter looks cleaner does not verify its processing capacity.

Routine maintenance protocol

1. Record tank volume, actual water removed and actual replacement volume.
2. Test the tank before the change when the result is meant to explain a trend.
3. Prepare and measure replacement water in a dedicated residue-free container.
4. Siphon into a light-coloured bucket and inspect it for juveniles before disposal.
5. Remove recoverable decomposing food and localized accumulations for a stated reason.
6. Do not strip every conditioned surface in the same maintenance event.
7. Refill, restore equipment and confirm flow, air and temperature.
8. Record animal distribution, feeding response and any loss without assigning a cause.

What the evidence does not prove

• Every Neocaridina tank needs 10, 20 or 50 percent changed each week.
• A zero-exchange biofilm experiment makes an unmeasured no-change aquarium safe.
• Large changes are inherently deadly or small changes are inherently harmless.
• A water change triggers a healthy molt or breeding.
• A top-off exports retained dissolved material.
• Old aquarium water contains the biological filter.
• A brief tap rinse instantly kills every useful organism in filter media.
• One nitrate or TDS number diagnoses a death or defines complete water quality.

Primary sources and authoritative measurement references

1. Viau, Marciano, Iriel and López Greco, 2016. Assessment of a biofilm-based culture system within zero water exchange. Aquaculture Research 47(8): 2528-2542.
2. Vazquez, Delevati-Colpo, Sganga and López Greco, 2017. Density and gender segregation effects in the culture of the caridean ornamental red cherry shrimp. Journal of Crustacean Biology 37(4): 367-373.
3. U.S. Geological Survey. A Primer on Water Quality. Evaporation and concentration of dissolved minerals.
4. Melo, 2023. Efeito de compostos nitrogenados no camarão Neocaridina davidi e na rã Aquarana catesbeiana. Doctoral thesis, Federal University of Minas Gerais.
5. U.S. Environmental Protection Agency. Ammonia. Measurement, speciation and effects of pH and temperature.
6. U.S. Centers for Disease Control and Prevention, 2024. About Water Disinfection with Chlorine and Chloramine.
7. Bagchi et al., 2014. Temporal and Spatial Stability of Ammonia-Oxidizing Archaea and Bacteria in Aquarium Biofilters. PLOS ONE 9(12): e113515.
8. McKnight and Neufeld, 2024. Comammox Nitrospira among dominant ammonia oxidizers within aquarium biofilter microbial communities. Applied and Environmental Microbiology 90(7): e00104-24.
9. Wert, Neemann, Rexing and Zegers, 2008. Biofiltration for removal of biodegradable organic matter and residual ammonia. Water Research 42(1-2): 372-378.

Naming note: older papers may use Neocaridina heteropoda for the animal now commonly treated as Neocaridina davidi. See the dated naming standard.