Guppy Tank Size: How Many Guppies in 5, 10, 20 and 29 Gallons
Published: 2026-11-15 · FishyKart Blog
The minimum guppy tank size is 5 gallons for one adult guppy plus 2 gallons for each additional guppy, the rule in the PetMD guppy care sheet reviewed by veterinarian Maria Zayas, DVM. That works out to 1 guppy in 5 gallons, 3 in 10 gallons, 8 in 20 gallons and 13 in a 29 gallon. Because Poecilia reticulata gives birth to live fry every few weeks, a mixed group outgrows those numbers fast unless you plan for the babies.
Photo: Marrabbio2 - Wikimedia Commons, CC BY-SA 3.0
How Many Guppies per Tank Size
| Tank | Litres | Adult guppies (5 + 2 rule) | Example group |
| 5 gallon | 19 L | 1 | 1 male |
| 10 gallon | 38 L | 3 | 1 male + 2 females, or 3 males |
| 15 gallon | 57 L | 6 | 2 males + 4 females |
| 20 gallon | 76 L | 8 | 2 males + 6 females |
| 29 gallon | 110 L | 13 | 4 males + 9 females |
| 40 gallon | 151 L | 18 | 5 males + 13 females |
| 55 gallon | 208 L | 26 | 7 males + 19 females |
Our calculation from the PetMD guppy care sheet: guppies = (gallons - 5) / 2 + 1, rounded down. The groups follow PetMD's ratio of two to three females per male. The counts are for adult guppies only, with nothing else in the tank; each tank mate takes away space.
Why the Rule Is 5 Gallons Plus 2
An adult guppy is small, up to about 2 inches with females larger than males, so the volume is not about body size. It covers three things: stable water (a small tank changes temperature and ammonia faster), swimming space for an active fish, and room for males to display without one female getting all the attention. Guppies want 72-82 F (22-28 C) and pH 6.8-7.8, which is easier to hold in more water. More on the temperature side in our guppy water temperature guide.
The Fry Problem: Tanks Fill Up by Themselves
A mixed-sex guppy tank does not stay at the number you bought. A female gives birth after a gestation of roughly 21-35 days to about 20-80 live fry, and she can store sperm and drop several broods without a male present (see pregnant guppy). Our rough example: a 10 gallon trio with 2 females, each dropping 3 broods of 20 fry in about three months, can produce around 120 fry, roughly 40 times what the tank is sized for. In practice adults eat many fry, but enough survive to overstock a small tank within a few months. Your options:
- Males only. No fry, the brightest colors, and the full count from the table. Males chase each other less when there are no females to compete for.
- Females only. Calm, but females bought from a mixed tank may already be pregnant.
- Mixed with a plan. A bigger tank than the table suggests, a buyer or second tank for young fish, and dense plants for cover.
How to tell the sexes apart before you buy: male vs female guppy.
5, 10 or 20 Gallons: Which to Choose
- 5 gallon: one male guppy, as a desk tank. Too small for a group and for fry. Better 5 gallon options are in best fish for a 5 gallon tank.
- 10 gallon: a trio or 3 males. The smallest size we would call a guppy tank. Our how many fish in a 10 gallon tank guide shows mixed stocking.
- 20 gallon long: the most forgiving choice for a breeding group of 8, with space for floating plants where fry hide. Size and weight are in 20 gallon tank dimensions.
- 29 gallon and up: a colony with fry growing out, or guppies as part of a community.
When to Stock Fewer Guppies
Use fewer fish than the table if you keep other species with the guppies, if the filter is small or you change water less than every week or two, or if the tank is new and still cycling. Fancy long-tailed males also do better without fast or nippy tank mates; see guppy tank mates before adding anything. The sign you have too many: nitrate climbing between water changes, cloudy water or females hiding all day.
Starting from scratch? Our guppy care guide covers food, water and breeding, and fishykart.com stocks guppies in its guppy collection.