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When people ask “How many fish per species should I keep?” they are usually talking about shoaling or schooling fish. The short answer is: most small community fish are happier, braver, and healthier in groups, and the number you choose matters a lot. This guide explains why group size is so important, gives beginner-friendly numbers for popular species, and shows you how to plan a tank that lets fish behave naturally. You will also learn when fewer fish is better, how to read fish behavior to check if your shoal is big enough, and how to build a peaceful, stable community.
What Shoaling Means, And Why Numbers Matter
Shoaling means fish prefer to live in a social group of the same species. Schooling is a tighter, synchronized version of shoaling, often seen when fish feel threatened or are in strong current. In both cases, being in a group helps fish feel safe. They explore more, eat better, show brighter colors, and display natural behavior. When kept in numbers that are too small, they can become shy, hide all day, nip fins, or bully weaker individuals.
In short, numbers are not just a decoration choice. The size of the group supports mental health, reduces stress hormones, and can prevent many behavior problems. In many tanks, one good-sized group of a single species looks far better and lives far better than several undersized groups.
The Three Pillars Of Stocking: Space, Social Needs, Filtration
Good stocking is a balance of three pillars. First is space: the tank must be large enough for the fish at their adult size, with swimming room and hiding spots. Second is social needs: a species that needs a group should have that group, not a token pair. Third is filtration and maintenance: your filter and your water-change routine must keep ammonia and nitrite at zero and nitrates low, even when the fish are fully grown.
Old rules like “one inch of fish per gallon” are unreliable. A better approach is to plan around adult size, activity level, territory needs, and your maintenance schedule. For most small shoalers, the baseline minimum is six, but many species do best at eight to twelve, and tiny nano fish may look and behave best at twelve to twenty. Bigger groups spread any nippiness and make shy fish bolder.
General Guidelines Before Choosing Numbers
Try to keep one large shoal rather than multiple tiny ones. Each species needs room to form stable social ranks. Mixing many small groups in a small tank creates stress and dull behavior.
Match water parameters and temperature. Some shoalers like cool water, others like it warm. Keep fish that want the same range. This keeps immunity strong and reduces stress.
Think in layers. Many aquariums have a top swimmer group, a midwater shoal, and a bottom group like corydoras or loaches. This spreads activity and reduces conflict for space.
Plan for adults. Buy juveniles knowing they will grow. Tiger barbs, rainbowfish, many loaches, and cichlids change a lot as they mature. Pick a group size that still works later.
Minimum Group Sizes By Category And Popular Species
Below are beginner-friendly numbers for common aquarium fish. These assume a tank that is appropriately sized and filtered, with matching water parameters and suitable companions. When in doubt, go bigger on the group and lighter on how many species you include.
Tetras (Characins)
Neon Tetra (Paracheirodon innesi): Keep 10 to 15. They are more confident and colorful when the group is large. Warm, soft water helps. Avoid mixing with fin-nippers.
Cardinal Tetra (Paracheirodon axelrodi): Keep 8 to 12. Slightly larger than neons and usually prefer warmer water. They look best as a large single-species shoal.
Rummy-Nose Tetra (Hemigrammus bleheri and relatives): Keep 10 to 12. Their tight schooling is a great stress barometer; if they spread out and lose nose color, check water quality.
Ember Tetra (Hyphessobrycon amandae): Keep 12 to 20. Very small and peaceful. A big group fills the midwater with warm color in nano tanks.
Black Skirt Tetra (Gymnocorymbus ternetzi): Keep 8 to 10. Can nip fins if kept in small groups; larger groups spread the behavior. Needs more room than most nano tetras.
Serpae Tetra (Hyphessobrycon eques): Keep 10 to 12 to reduce nipping. They are lively and can be pushy. Larger groups and a big tank help.
Rasboras
Harlequin Rasbora (Trigonostigma heteromorpha): Keep 8 to 12. Hardy, peaceful, and excellent for community tanks. Provide open swimming area and some plants.
Chili Rasbora (Boraras brigittae): Keep 12 to 20. Tiny and shy if kept too few. A big group brings them out and looks beautiful in blackwater-style setups.
Lambchop Rasbora (Trigonostigma espei): Keep 8 to 12. Similar to harlequins but slimmer; great for medium planted tanks.
Danios And Minnows
Zebra Danio (Danio rerio): Keep 6 to 10. Very active and like long tanks. They can bother slow fish if bored, so space and group size matter.
Celestial Pearl Danio (Danio margaritatus, CPD): Keep 8 to 12. Gentle but timid. Dense plants and a good group bring out natural displays.
White Cloud Mountain Minnow (Tanichthys albonubes): Keep 8 to 12. Prefer cooler water. Great for unheated tanks and peaceful communities.
Barbs
Tiger Barb (Puntigrus tetrazona): Keep 8 to 12 minimum. In small numbers they are relentless fin-nippers. In larger shoals with space they focus on each other. Avoid slow, long-finned tankmates.
Cherry Barb (Puntius titteya): Keep 8 to 12 with more females than males, ideally one male to two or three females. Peaceful and nice in planted tanks.
Livebearers
Guppies and Endlers (Poecilia reticulata and hybrids): Start with 6 or more. Use a ratio of one male to two or three females to reduce chasing, or keep an all-male group in a larger number to spread sparring. They prefer hard, alkaline water.
Platies (Xiphophorus maculatus): Keep 6 or more. Use the same male-to-female ratio as guppies. Hardy and active; excellent for beginners.
Mollies (Poecilia sphenops and relatives): Keep 6 or more. They get bigger and produce more waste than guppies; they prefer hard, alkaline water and sometimes a touch of salt. Provide plenty of space.
Swordtails (Xiphophorus hellerii): Keep 6 or more, one male to several females. Males can be territorial; larger tanks help.
Corydoras And Other Catfish
Corydoras (many species): Keep 8 or more of the same species. Mixed corys are cute but they bond best with their own kind. Soft substrate like sand helps whiskers stay healthy.
Otocinclus Catfish (Otocinclus spp.): Keep 6 to 10. Gentle algae grazers that need a mature tank with biofilm and frequent small feedings. Never keep a lonely oto.
Bristlenose Pleco (Ancistrus spp.): Usually keep one per tank unless breeding or in very large tanks with many hides. They are not shoalers. Feed wood and veggies.
Loaches
Kuhli Loach (Pangio spp.): Keep 6 to 12. Shy but fun at night. Sand substrate, leaf litter, and caves make them feel safe.
Dwarf Chain Loach (Ambastaia sidthimunki): Keep 8 to 12. Active snail-eaters. Provide line-of-sight breaks and a lid; they are lively.
Clown Loach (Chromobotia macracanthus): Keep 6 or more, but they need a very large tank as adults. Do not buy for small aquariums.
Rainbowfish
Dwarf Neon Rainbow (Melanotaenia praecox): Keep 6 to 10 with both sexes. They are active and like long tanks with flow. Good water quality brings out color.
Boesemani Rainbow (Melanotaenia boesemani) and similar larger species: Keep 8 to 12 in large tanks. They are strong swimmers and need space.
Threadfin Rainbow (Iriatherina werneri): Keep 8 to 12 with more females than males to reduce fin flaring. Gentle and beautiful, best with peaceful tankmates.
Gouramis And Labyrinth Fish
Honey Gourami (Trichogaster chuna): Keep a trio if space allows, one male with two females, or a small group in larger planted tanks. Very peaceful.
Sparkling Gourami (Trichopsis pumila): Keep 6 or more. Small but feisty; dense plants help line-of-sight breaks.
Dwarf Gourami (Trichogaster lalius): Usually one male per tank. Males may fight and can harass smaller fish in tight quarters.
Betta Splendens: Keep one per tank unless you are experienced. Female “sororities” are risky, require a large group and heavy cover, and are not recommended for beginners.
Cichlids
Angelfish (Pterophyllum scalare): Start with 5 to 6 juveniles in a large, tall tank, then rehome extras once pairs form. Not a shoaler, but doing this reduces bullying while young.
Discus (Symphysodon spp.): Keep 5 to 7 in a mature, warm, stable tank. They are social but demanding; best for experienced keepers.
Ram Cichlids (Mikrogeophagus ramirezi) and Bolivian Rams (M. altispinosus): Keep as a pair or, in larger tanks, one male to two females with many hides. They are not shoalers.
Apistogramma: Often one male with two to three females in a well-structured tank. Not a shoaler; focus on territories and cover.
Mbuna And Other African Rift Lake Cichlids: Strategies vary by species. Many keep larger groups of a single species or overstock the tank with careful filtration to spread aggression. Research each species before buying.
Picking Numbers For Your Tank Size
The best way to choose numbers is to pick your showcase species first, then build around them. Decide if you want a busy midwater shoal, a nano display, or a calm, planted scape. Below are examples to inspire you.
10-Gallon Nano
Choose one main shoal and a small bottom group. For example, 12 to 16 chili rasboras with 6 to 8 pygmy corydoras. Keep maintenance steady and provide plants. Avoid mixing many species; let one group shine.
20-Gallon Long
Great footprint for small shoalers. A classic setup is 10 to 12 harlequin rasboras, 8 to 10 neon tetras, and 8 to 10 corydoras of one species. For a simpler approach, pick one midwater shoal of 12 to 16 and one bottom group of 8 to 10.
29-Gallon Planted
Try 10 to 12 rummy-nose tetras, 8 to 10 otocinclus, and a trio of honey gouramis. Or focus on one large shoal like 15 ember tetras plus 10 corydoras and leave it at that. The larger shoal will look fantastic.
55-Gallon Community
You have room for one or two large shoals. Consider 15 to 20 cardinals, 12 to 15 panda corydoras, and a group of 8 dwarf neon rainbowfish. Or build a barb tank with 12 to 15 tiger barbs and a separate shoal of 12 to spread energy across the tank. Keep decorations that break sight lines to reduce chasing.
Reading Behavior: Do You Have Enough Fish?
If a shoaler is kept in too small a group, you may see constant hiding, colors washed out, jumpiness, fin-nipping, or a single dominant fish harassing others. If the group is big enough, the fish explore, colors are rich, and the group spreads out and forms loose patterns. You should see natural behaviors like displaying, foraging, and calm resting. If your fish look nervous, add numbers of the same species first before adding new species, as long as your filtration and tank size allow it.
When Fewer Is Better
Not all fish want a crowd. Territorial or solitary species do better alone or in specific ratios. Bettas are best single. Dwarf gourami males often fight. Many cichlids form pairs and defend areas. Bristlenose plecos can quarrel without enough caves. For livebearers, too many males with too few females leads to stress; fix this by using a one male to two or three females ratio or keeping a larger all-male group where mild sparring is spread out.
Water Parameters And Temperatures Matter
Group size cannot fix a mismatch in water requirements. White cloud minnows like cooler water, while cardinals and rummy-nose prefer warm, soft, acidic water. Livebearers like guppies and mollies prefer hard, alkaline water. Keep species together that want the same conditions and your shoals will behave naturally and live longer.
Aquascape For Shoals
Shoalers need both open water and cover. Use plants, driftwood, and rock to create a few long sight lines for swimming, plus broken lines of sight to reduce chasing. Dark substrates and floating plants can calm shy shoalers. Provide caves and leaf litter for bottom dwellers like corydoras and kuhli loaches. A well-structured tank reduces stress and helps groups maintain stable social ranks.
Filtration, Flow, And Maintenance
More fish means more waste. A filter that turns the tank volume over 5 to 8 times per hour is a good starting point for most communities. Some shoalers, like danios and rainbowfish, enjoy moderate flow; others prefer gentle water. Keep ammonia and nitrite at zero and try to keep nitrate under 20 to 30 ppm by doing regular water changes. Do not increase group size faster than your maintenance routine.
Quarantine And Sourcing Groups
When building a shoal, try to buy from the same source and similar size if you can. This reduces conflict and disease risk. If you must mix sources, quarantine new fish for a few weeks to protect your main tank. It is better to add to the same species you already keep than to split your numbers across many species.
Mixing Species: Will They School Together?
Most shoalers prefer their own species. Neons and cardinals may swim near each other, but they will not form a stable mixed school. Rummy-nose variants often mix among themselves because they are closely related. For the best behavior, pick one species and give it the numbers it needs. If you want variety, choose a different layer of the tank rather than a second small midwater group.
Common Pitfalls To Avoid
Do not keep three or four of a shoaling species and expect calm behavior. This is the most common mistake. Do not mix fast nippy fish with slow long-finned species. Do not buy large shoalers like clown loaches or big rainbowfish for small tanks. Do not rely on the “inch per gallon” rule. Do not forget adult size and sex ratios, especially with livebearers, gouramis, and cichlids.
How To Adjust If You Already Undershot The Number
If your fish are shy or nippy and you realize the group is too small, first check your water quality. If it is good and your tank can handle more fish, add more of the same species to reach the recommended range. Rearrange some decor right before adding the new fish to reduce territorial behavior. If your tank is already at its limit, consider rehoming a different species to make room for proper shoal numbers. Your fish will be healthier for it.
Species That Often Need Even Bigger Groups
Some fish are especially social and really bloom in large numbers. Examples are ember tetras, chili rasboras, rummy-nose tetras, and corydoras. For these, going from six to twelve or more can transform the tank. Conversely, species with pushy tendencies like tiger barbs or serpae tetras become safer in larger groups because the attention is spread out. If your tank allows, choose a number at the high end of the recommended range.
Realistic Stocking Examples With Numbers
Beginner planted 20-long: 12 harlequin rasboras as the main shoal, 10 panda corydoras on the bottom, and a honey gourami trio. The rasboras provide movement, the corys keep the bottom lively, and the gouramis add calm color at the top.
Nano blackwater 10-gallon: 16 ember tetras and 8 pygmy corydoras. Keep leaves and botanicals for a cozy look. The larger ember group shows rich orange and frequent foraging.
Active 55-gallon river look: 15 rummy-nose tetras, 10 zebra danios, and 12 corydoras sterbai. Provide flow and open lanes for the danios, and a sand patch for the corys. The rummy-nose school tightens and loosens with the current.
Hard-water livebearer tank 29-gallon: 8 male guppies for color without fry, 10 platies with a one male to two or three females ratio, and 8 otocinclus once the tank is mature. Plant heavily and keep up on water changes.
Signs You Picked The Right Number
Your fish are visible, feeding well, and showing natural colors. Shoals spread out and reform smoothly. There is little to no fin damage. You do not see a single fish constantly chasing others. The aquarium looks calm even when busy. Your water tests are stable and your maintenance routine feels manageable. If you can tick these boxes, your group sizes are likely a good fit.
Frequently Asked Questions
Can I keep just six of everything? Six is a common baseline, but many species act better at eight to twelve or more. If your tank is small, pick fewer species and give each the number it needs.
Will two shoals split the tank nicely? Sometimes, but often one species dominates. It usually looks better to do one big midwater shoal and one bottom group, rather than two small midwater shoals.
Can I add numbers slowly? Yes. Add in small batches, test water, and watch behavior. Just try to reach the target group size within a reasonable time so fish are not left under-socialized.
Do fish of the same genus form one group? Usually no. They may swim near each other but still prefer their own species. For example, neon and cardinal tetras rarely form one stable school.
Is overstocking ever okay to reduce aggression? Some cichlid keepers use controlled overstocking with strong filtration and heavy maintenance to spread aggression. This is an advanced method and not needed for most community shoalers. For beginners, stock conservatively and choose proper group sizes instead.
Simple Planning Checklist
Pick your showcase species first, then commit to a proper group size. Match water parameters for all species. Choose one shoal per layer if possible: midwater and bottom. Size the tank for adult fish and leave swimming space. Provide hides and sight breaks. Set filtration and water changes to handle the full adult bioload. Quarantine new fish and add numbers of the same species before adding new species. Watch behavior and adjust as needed.
Conclusion
How many fish per species should you keep? For most shoaling fish, more than you think. Groups of eight to twelve, and sometimes twelve to twenty for tiny nanos, unlock natural behavior, brighter colors, and calmer tanks. Resist the urge to collect too many species. Instead, pick one or two favorites and give them a proper community of their own kind. Plan around adult size, match water parameters, and support your fish with good aquascaping and solid maintenance. When you get the numbers right, your aquarium looks better and your fish live better. That is the heart of a successful shoaling tank.