How many fish can I put in a 10-gallon tank?

A 10-gallon tank can comfortably support around 15 bioload points with standard filtration. That equates to roughly 6–8 small fish like neon tetras or ember tetras. Avoid anything larger than 2 inches in a 10-gallon, and always account for school minimums — most schooling fish need at least 6 to reduce stress.

What is the 1-inch-per-gallon rule and is it accurate?

The 1-inch-per-gallon rule states that you can keep one inch of adult fish per gallon of water. While simple, it is widely considered outdated and inaccurate by modern aquarists. It ignores bioload (waste production), fish body shape, swimming space needs, oxygen consumption, and filtration quality. A bioload-based approach — as used by our calculator — gives a far more reliable stocking estimate.

What is bioload in a fish tank?

Bioload refers to the biological waste a fish produces — ammonia from respiration and excretion — and the resulting burden placed on your tank's nitrogen cycle. Large, messy fish like goldfish and cichlids have a high bioload relative to their size, while small fish like neon tetras have a much lower bioload. Overstocking bioload overwhelms the beneficial bacteria in your filter, leading to ammonia and nitrite spikes that can kill fish.

Can I keep a betta with other fish?

Male bettas should generally be kept alone or with very carefully chosen tankmates. They are aggressive toward other bettas and will attack fish with flowing fins such as guppies and gouramis. Suitable companions in a larger tank (10+ gallons) include bottom-dwellers like corydoras, fast-moving schooling fish, and snails. Female bettas can sometimes be kept in groups of 5 or more in a 'sorority tank', though this requires careful monitoring.

What happens if a fish tank is overstocked?

Overstocking leads to elevated ammonia and nitrite levels as your filter cannot process the waste load fast enough. This causes stress, suppressed immune systems, fin damage, and in severe cases fish deaths. Oxygen depletion is another risk in heavily overstocked tanks, particularly at night when plants are not photosynthesising. Regular water testing and partial water changes are essential if you are running close to stocking limits.

How does filtration affect how many fish I can keep?

Filtration quality directly determines your tank's biological processing capacity. An oversized canister filter or a heavily planted tank with supplemental CO2 can support significantly more bioload than a basic hang-on-back filter on the same volume. Our calculator applies a filtration multiplier ranging from 0.9× for under-filtered tanks to 1.3× for sumps and advanced planted setups.

What fish are best for a 55-gallon community tank?

A 55-gallon tank is ideal for a lively community setup. Popular combinations include a school of 8–10 rummynose or cardinal tetras, a group of 6 corydoras along the bottom, a pair of dwarf gouramis as a centrepiece, and a bristlenose pleco for algae control. Avoid mixing aggressive species like tiger barbs or serpae tetras with slow-finned fish. Total bioload should stay under 80% of your effective tank capacity.

Do schooling fish need to be kept in groups?

Yes. Schooling fish are shoaling species that experience significant stress when kept in numbers below their minimum group size. A lone neon tetra, for example, will become timid, may stop eating, and is prone to disease. Most schooling species require a minimum of 6 individuals, with 8–12 being ideal to see natural schooling behaviour. Our calculator flags school-size warnings when you fall below the recommended minimum.

Aquarium Stocking Calculator

Stocking an aquarium is one of the most common areas where hobbyists make costly mistakes — not out of carelessness, but because the information available online is fragmented, contradictory, and often based on outdated rules of thumb that do not reflect how modern fishkeeping actually works. The old one-inch-per-gallon rule, still repeated across forums and pet shop advice, ignores everything that actually determines whether a tank can support a given fish: the fish’s adult size, its waste production, its swimming behaviour, its aggression toward tankmates, its schooling requirements, and the quality and size of your filtration. A single adult oscar produces more biological waste than twenty neon tetras, yet the inch-per-gallon rule treats them identically. This calculator replaces that outdated approach entirely.

The aquarium stocking calculator uses a bioload-based method covering over four hundred freshwater, saltwater, and brackish species. Every species in the database has individual values for adult size, waste production rate, minimum school size where applicable, aggression profile, and compatibility with other species. The calculator checks your entire fish community simultaneously — not just whether the total bioload fits within your tank’s capacity, but whether the specific combination of species you have chosen can coexist without territorial conflict, predation risk, or parameter incompatibility.

Use the ProHobby™ Advanced Aquarium Stocking Calculator to determine how many fish your tank can safely support based on bioload, filtration, and tank conditions. With 400+ fish species presets, choice of 3 water types, real-time compatibility checks, and detailed FAQs, it helps you avoid costly stocking mistakes — completely free, with no signup required.

Designed as an all-in-one, easy-to-use tool for real-world aquariums!

Bioload is the total biological demand your fish place on your filtration system — specifically, how much ammonia they produce through respiration and waste, which your filter’s beneficial bacteria must convert to nitrite and then to nitrate. Different fish produce vastly different amounts of ammonia per unit of body weight. Goldfish and oscars are famously high-waste producers, which is why a single goldfish in a small tank causes persistent water quality problems that a dozen small tetras in the same volume would not. Cichlids, large catfish, and most large-bodied fish sit at the high-waste end of the spectrum. Small schooling fish such as rasboras, small tetras, and pygmy corydoras are low-waste producers that pack well into a bioload calculation without stressing filtration.

The calculator applies a filtration quality adjustment that reflects a real-world variable most stocking tools ignore entirely. A heavily planted tank with a high-quality canister filter and regular maintenance can support a meaningfully higher bioload than the same volume with an undersized hang-on-back filter and infrequent water changes. The adjustment does not allow you to overstock dangerously, but it does give credit for the genuine biological capacity that good filtration and plant growth provide.

Schooling fish present a particular challenge in stocking calculations. A single neon tetra added to an already-stocked tank is not one fish — it is a welfare problem, because neon tetras kept in groups smaller than six exhibit chronic stress, suppressed immune function, and shortened lifespans. The calculator enforces minimum school sizes for all schooling and shoaling species, warning you when a group is too small and refusing to count a lone schooling fish as adequately stocked. This reflects actual fish welfare standards rather than just physical tank capacity.

Compatibility warnings appear in real time as you build your fish list. The database tracks aggression profiles and flags known problem combinations: fin-nipping species added to tanks with long-finned fish, cichlids paired with small community fish they will predate, multiple male bettas, territory-competing species of similar appearance, and predator-prey size relationships where one fish is large enough to eat another. These warnings are specific to the species combination you have entered, not generic caution text. A tiger barb with a betta is flagged. A tiger barb with a bristlenose pleco is not.

The calculator supports freshwater, saltwater, and brackish communities in separate modes. Saltwater fish stocking works differently from freshwater — marine fish are generally more territorial, more sensitive to water quality fluctuations, and interact differently with live rock and reef inhabitants. The reef-safe indicator in the saltwater database identifies species that pose a risk to corals, invertebrates, or small ornamental shrimp. Brackish species such as figure-eight puffers, mudskippers, and Colombian shark catfish are included with their specific salinity tolerance ranges and compatibility notes, as brackish stocking is an area almost entirely absent from most online stocking resources.

Once you have built your stocking list, use the volume figure from our aquarium tank volume calculator to ensure the capacity number you are stocking against reflects your actual usable water volume — not the tank’s nominal size — and the flow and filtration calculator to confirm your filter is rated to handle the bioload the stocking calculator has calculated for your specific community.