Aquarium Stocking and Tankmates: How Many Fish, and Which Ones Go Together

Every fish and shrimp page here gives a minimum tank size and then stops. That leaves the two questions a new keeper actually has once the tank is bought: how many fish can go in it past that floor, and which species can share it. Neither has a single number, which is why the species pages never carried one. This page covers what the popular rule gets wrong, the three things that set a tank's real capacity, the four checks that decide whether two species belong together, the group sizes the welfare research supports, how to get new fish from the bag into the tank and add them without a fight, and a table of every fish and shrimp on the site with its floor and its known bad pairings.

What the Inch-per-Gallon Rule Gets Wrong

The rule everyone learns first is one inch of fish for every gallon of water. The honest version: the rule works for small community fish of 1 to 3 inches and fails for larger species, because a 10-inch fish needs vastly more space than ten 1-inch fish. The reason is that a fish's waste and its swimming room both scale with its bulk, not its length, and a fish twice as long is far more than twice the animal. The rule also counts the fish at the size it was bought, and a juvenile is bought small.

The older veterinary version of the rule is about surface area rather than volume. It allows one inch of freshwater fish for every 12 square inches of surface area, which at least captures that oxygen enters at the surface and a tall narrow tank holds fewer fish than a long low one of the same gallons. It has the same limit, though: it is a rule for small fish, and it says nothing about temperament.

The Three Limits That Replace It

Three factors replace the rule. Waste load: fish eat, produce waste, and the bacteria in a cycled filter and any live plants break it down, so the stock a tank can carry is set by how much waste the filter handles, not by the gallons. Swimming space: each fish needs room in proportion to its adult size and how active it is, and a fancy goldfish's 20-gallon long is sized for a 30-inch length and a 12-inch turning radius while an angelfish's 29 gallons is sized for its 8-inch height. Aggression: a peaceful community fish and an African cichlid need different stocking plans in the same tank.

The first limit is the one you can measure. The filtration guide explains why the filter's bacteria colony is the tank's real capacity, and the water changes guide gives the test that shows when that capacity is exceeded: nitrate before the weekly change. Nitrate belongs below 40 ppm under regular monitoring. A tank that sits under 20 ppm before its change has room for more waste. A tank drifting toward 40 is full, whatever the gallons say.

The practical rule that falls out of this: understock the aquarium at first and add more fish later if the numbers allow. Stock the schooling fish at their minimum number, wait two weeks, test, and add the next group only if the reading held.

The Four Checks Before Two Species Share a Tank

A tankmate pairing has to pass four checks, and the ones that fail in practice usually pass three.

• Temperature overlap. The two species' ranges have to share a band the tank can sit in all year. The tetra pages settle one common case: cardinals tolerate water to 86°F, while neons do best at 78°F or below and top out around 81°F, so cardinals are the tetra for a warm discus tank.
• Water chemistry overlap. Soft acidic and hard alkaline water each have their fish, and the pH, GH and KH guide is the place to find out which your tap delivers before choosing. Fish chosen to match the water outlast water forced to match the fish.
• Size in both directions. Anything that fits in a mouth ends up there, and aggression toward new fish is common among freshwater cichlids and other fish over 3 inches. The ghost shrimp tank setup guide makes the point that the check runs both ways: a ghost shrimp is prey to a betta as a juvenile and a predator of cherry shrimp as an adult.
• Temperament against body shape. A nipper and a long-finned fish never work, whatever else matches. The betta shows it in one line: no known fin-nippers like tiger barbs, and no long-finned species it may spar with.

Passing all four is the whole test. The usual workable tank mates for a betta, corydoras catfish, mystery snails, ghost shrimp and some small tetras or rasboras, are the ones that pass: warm-water, peaceful, short-finned, and either too big to eat or too fast to catch.

Group Size: Why Six Is a Floor and Ten Is Better

Every schooling species page here says six minimum, and the research the advice rests on points higher. Saxby, Adams, Snellgrove, Wilson and Sloman tested four common aquarium fish at different group sizes for a 2010 paper in Applied Animal Behaviour Science. Neon tetras and white cloud mountain minnows showed reduced aggression and darting and spent more time shoaling in larger groups. Angelfish and tiger barbs were more variable, though larger groups shoaled more, and the analysis showed improved welfare in larger groups of neons, white clouds and tiger barbs, with no clear link between group size and welfare in angelfish.

The study tested specific group sizes: the tetras and minnows were kept in groups of one, two, five and 10, and the groups of 10 had the greatest tendency to shoal. That is the number behind the neon tetra enrichment guide's case for a bigger school, and it settles the trade a new keeper faces at the shop: 10 of one species beats five each of two. The corydoras tank setup guide puts the same rule as one species of eight over two species of four.

The exception cuts the other way. For a fish that spreads its aggression across the group, numbers dilute it. Tiger barbs are the standard case: the same study found their welfare improved in larger groups, so a barb tank wants a bigger school, not a smaller one.

Getting New Fish From the Bag Into the Tank

Float the sealed bag on the tank's surface for 20 to 30 minutes so the water inside comes to the tank's temperature. That step handles heat. The bag water itself is the second problem. In transit the fish keep breathing into a sealed volume: carbon dioxide builds up, which is why the pH in a shipping bag often drops, and total ammonia climbs the longer the trip and the more fish share the bag. The low pH is doing the fish a favor, because ammonia at a lower pH sits mostly in its less toxic form. As pH and temperature rise, the balance shifts toward the toxic form. A fish that spent a day in a bag is sitting in ammonia that the bag's own chemistry is holding in check.

That is the reason the bag water never goes into the tank. Once the temperature matches, lift the fish out with a net, or pour the bag through a net held over a bucket, and release only the fish. Pouring the bag in adds the shop's ammonia, and whatever the shop's tanks were carrying, to water that had neither.

Slow mixing, the drip or the cup-at-a-time method, earns its time in two cases. The first is temperature: when the bag and the tank differ by more than 10°F, acclimate slowly, and past a gap of about 18°F, stretch it over two hours or more. The method is the same at any scale: put the bag or a bucket beside the tank, run an airline into it for oxygen, and add tank water to it gradually, so temperature and pH change together and slowly. The second case is chemistry, when the shop's water and yours sit at very different pH or hardness. A fish going from one tap to a similar tap across town needs the float and the net, and little else.

Numbers matter as much as method. A newly cycled tank's bacteria colony is sized to the waste it has been fed so far. There are two safe ways to stock it: 1 or 2 fish at a time over several weeks, or the full stock at once with the water tested every day until it settles. A schooling species suits the second route better, because a pair of neons is the one stocking choice with no defense. After that first group, the nitrate rule above sets the pace for every addition.

Adding New Fish Without a Fight

An established tank has an owner, and adding to it is about territory. Decorative objects divide the aquarium and let fish hold their own space, which matters most when space and territory are at a premium. When the new fish arrive, rearrange the decor while the bag floats, which breaks up the established territorial markers, and feed the resident fish at the moment the new ones are released. Both moves give a newcomer an hour without a defender.

The manual's other warning is the one that matters more: there is significant risk of introducing disease if the new fish have not been carefully quarantined. The quarantine guide covers the tank and the timeline. A new fish that skips it brings the shop's tank to yours.

Order matters too. The angelfish handling guide has angelfish added last, after the community is established, because an adult that has held the tank alone treats every later arrival as an intruder, and one over 3 inches eats a neon tetra it was raised with.

Species by Species

Each floor below is the figure from the species' own tank setup guide, which wins where the two disagree. The last column is the pairing that page or the linked guide warns against.

What Goes Wrong

• Stocking for the size in the bag. A 1-inch oscar or a 2-inch angelfish passes every rule at the shop. Both are counted at adult size or the tank is overstocked in a year.
• Two half-schools instead of one. Six neons and six rasboras looks like variety and gives each species a group at the floor. Ten of one is the better tank and the calmer one.
• A pairing that passed three checks. The temperature matched, the water matched, the sizes matched, and the barbs took the betta's tail apart in a week.
• Adding to a full tank because the fish look fine. Nitrate creeping toward 40 ppm is the tank saying no. The sick fish guide sees the result a month later.

The Takeaway

Take the species page's minimum as a floor, count every fish at adult size, and let nitrate say whether there is room for more: under 20 ppm before the weekly change means yes, past 40 means the tank is full. Before any two species share a tank, check temperature, water, size in both directions and fins against nippers. Buy schooling fish in tens, not sixes, and one species at a time.

Sources & Further Reading