How many Neocaridina can a tank hold?
Evidence review: August 12, 2026
There is no evidence-backed universal number of Neocaridina davidi per gallon. Aquarium volume is one system measurement. It does not describe animal age, food, conditioned surface, oxygen, filtration, water exchange or the outcome you are trying to protect. Use the filtration guide before turning filter ratings or bubbles into capacity. Do not add animals from an algae-control formula; grazing does not establish a cleanup dose.
The short version: use a starter count as a transparent procurement choice. Use density research to understand a tested grow-out condition. Set your own tank's operating limit from repeated growth, recruitment, loss, feeding and water-processing records.
What the density experiment actually tested
A controlled study reared newly hatched shrimp for 90 days at 2.5, 5 and 10 individuals per litre. The results report 24 experimental units: eight, nine and seven 2 litre vessels containing 5, 10 and 20 shrimp, respectively. Each vessel also contained 0.7 g of Java moss. The researchers used continuous aeration, 27 +/- 1 C water, daily formulated food scaled to the vessel's shrimp biomass and complete weekly water replacement. New offspring were removed so the assigned density stayed fixed.1
| Density | Shrimp per 2 L vessel | Vessels | 90-day result |
|---|---|---|---|
| 2.5/L | 5 | 8 | 92.50% model-estimated survival and the largest mean male size |
| 5/L | 10 | 9 | 93.33% model-estimated survival and an intermediate growth result |
| 10/L | 20 | 7 | 87.14% model-estimated survival and lower mean growth |
The survival percentages are estimates from the authors' logistic model, and survival was not statistically different among treatments. By day 90, females at 2.5/L were 45% heavier than females at 10/L. Males at 2.5/L were also heavier than males at both higher densities. Females became ovigerous at every density, but egg number, hatch output and recruitment were not reported, and newborns were removed.1
One vessel mean is one observation
The shrimp in each density vessel were weighed together every 30 days. The repeated growth response was therefore the vessel's mean mass, not a trajectory for every shrimp. Eight vessels at 2.5/L means eight replicate growth histories, not 40 independent histories. The 9 vessels at 5/L and 7 vessels at 10/L are the same kind of unit.
At the end, biochemical assays used all available animals from a vessel, and the researchers pooled vessels when one did not provide enough tissue. Those assays cannot repair the density replication or show which individual changed. The source also does not report brood or family contributions, actual food intake or why replicate counts differed among densities.
Why this does not become a per-gallon rule
Multiplying 10 shrimp/L by an aquarium's labelled volume would silently discard the 2 litre vessel, juvenile starting stage, vessel-scaled food, 0.7 g moss, continuous aeration, weekly complete water replacement, offspring removal, unequal replicate counts and 90-day endpoint. It would also change the question from juvenile vessel-mean growth to the long-term capacity of a mixed-age colony.
The experiment supports a narrower conclusion: higher tested density reduced growth under those conditions, while survival did not differ statistically. It did not establish a disease threshold, prove that breeding stops, or identify a numerical maximum for a home aquarium. Use a defined cohort and measurement method before comparing growth across tanks.
How many should you start with?
No controlled study found in this review establishes a minimum starter group. If animals are unsexed, a larger group can improve the chance of receiving at least one of each sex. The calculation requires an assumption.
If each animal is independent and either sex is equally likely, the chance of at
least one of each is 1 - 2 x 0.5^n, where n is the group
size.
| Unsexed group | Chance under the 50:50 assumption |
|---|---|
| 4 | 87.50% |
| 6 | 96.88% |
| 8 | 99.22% |
| 10 | 99.80% |
Seller batches may not be random, independent or 50:50. This arithmetic does not predict maturity, fertility, relatedness, health, survival or breeding success. A reliably observed adult composition is more informative than assuming half of a purchase is female.
Measure an operating limit, not a maximum
A useful limit protects an outcome. It is not the greatest number that can remain alive. Count under a repeatable viewing method and record a wide range by life stage. Then track the range beside the following:
- Dated cohort growth and time to your own saleable-size gate
- Berried females, hatch observations and juveniles reaching later stages
- Deaths and missing cohorts with a stated observation window
- Food product, repeatable portion and what remains at the next observation
- Total ammonia nitrogen, nitrite and the normal maintenance schedule
- Temperature, aeration, pH, GH, KH and conductivity or TDS
Treat slower growth, reduced recruitment, unexplained loss, persistent food, lost nitrogen processing or unmanageable maintenance as signals to investigate. None identifies density by itself. Change one variable and keep the denominator.
When is a harvest sustainable?
Our practice
We do not clear a harvest from gallons or a calendar. A line needs a dated starting record, a hatch count or range, a cohort that reaches the photographed size gate, and an observed survival estimate. We then require repeated intervals in which recruitment replaces removals without deterioration in the selected growth, health and water-processing outcomes.
Until those records exist, any monthly population or revenue table is a business scenario, not a biological forecast.
If the problem is keeper capacity or responsible placement rather than a density experiment, use the surplus and rehoming guide. Do not reduce care while waiting for a destination.
What the evidence does not prove
- That one gallon supports a fixed number of shrimp
- That a visible crowd means disease or poor welfare
- That colonies stop breeding at a predictable count
- That removing animals is medically necessary at a fixed number
- That a 90-day juvenile trial predicts a multigeneration adult colony
Primary source
- Vazquez et al., 2017. Density and gender segregation effects in the culture of the caridean ornamental red cherry shrimp Neocaridina davidi. Journal of Crustacean Biology 37(4): 367-373.