Aquarium Stocking Calculator

Aquarium Stocking Guide

What do I need to know about The Inch-Per-Gallon Rule (and Its Limits)?

The classic '1 inch of fish per gallon' rule provides a rough starting point but has significant limitations. It does not account for fish body shape (a 6-inch Oscars needs far more space than a 6-inch neon tetra school), bioload (goldfish produce far more waste than equivalent-sized tetras), swimming patterns (active schooling fish need open swimming space), or aggression. Use it as a maximum ceiling, not a target to aim for.

What should I know about Bioload and Filtration?

Filtration is as important as tank volume. The nitrogen cycle (ammonia → nitrite → nitrate) depends on beneficial bacteria in your filter. A filter should turn over the full tank volume 4–10 times per hour. Heavily stocked tanks or messy fish (goldfish, cichlids, plecos) need the higher end. Never clean the filter and perform a large water change simultaneously — you risk crashing your cycle by removing too much bacteria at once.

What's the key thing to understand about Community Tank Compatibility?

Beyond numbers, fish compatibility matters enormously. Fin-nippers (tiger barbs, serpae tetras) should not be kept with long-finned fish. Aggressive cichlids need significantly more space than peaceful community fish. Schooling fish should be kept in groups of 6+ of the same species. Bottom-dwellers (corydoras, loaches), mid-water fish (tetras, barbs), and top-swimmers (hatchetfish, danios) can share a tank without competing for the same space.

What should I know about Cycling a New Tank?

Never add fish to an uncycled tank. The nitrogen cycle takes 4–8 weeks to establish. Signs of completion: ammonia and nitrite both read 0, nitrates present. Fishless cycling (using ammonia directly) is the most humane approach. Once cycled, add fish gradually — adding too many at once overwhelms the biological filter. The first signs of an overstocked or uncycled tank are fish gasping at the surface and ammonia or nitrite spikes.

What should I know about Why Fish Type Changes the Stocking Calculation So Much?

The fish_type input affects this calculator's stocking recommendations far beyond what fish size alone would suggest, because bioload — how much waste a fish produces and how much biological filtration capacity it demands — varies enormously between species of similar size. Messy, heavy-waste-producing fish such as large cichlids or goldfish need considerably more effective filtration and swimming space per centimeter of body length than an equivalently sized schooling fish like a small tetra, which tends to produce comparatively little waste. Beyond waste production, territorial or fin-nipping species also genuinely need more swimming space per individual regardless of their bioload, simply to reduce stress and aggression between tankmates sharing the same water volume. This is exactly why fish_type is kept as a separate input from fish_size_cm in this calculator rather than being folded into a single size-based rule — two fish of identical length can have very different real-world space and filtration requirements depending on what kind of fish they actually are.

What should I know about Surface Area and Oxygen Exchange?

Stocking a tank isn't purely a function of total water volume — gas exchange with the air happens specifically at the water's surface, where oxygen dissolves in and excess carbon dioxide (along with some ammonia off-gassing) escapes. This means a long, shallow tank with a large surface area relative to its volume can often support a healthier fish load than a tall, narrow tank holding the identical total number of litres but with much less surface area exposed to air. This factor becomes especially important in densely stocked tanks, in warmer water (which naturally holds less dissolved oxygen than cooler water), or in tanks without additional surface agitation from a filter outlet or air stone to help mix oxygen into the water column. When this calculator's max_inches or max_cm figure feels like it's pushing the upper end of what seems reasonable for your tank, checking that your tank shape provides generous surface area — and that your filter_quality setting reflects adequate water movement and surface agitation — is a useful sanity check beyond the raw volume number.

What should I know about Quarantining New Fish Before Adding Them to a Stocked Tank?

Adding a new fish directly into an established, fully stocked tank risks introducing disease or parasites that can spread quickly through fish that have no immunity to them. A separate quarantine tank — even a simple, inexpensive setup with its own filter and heater — lets new arrivals be observed for roughly 2-4 weeks before joining the main tank, giving time for common issues like ich, fin rot, or internal parasites to become visible and be treated without risking the rest of the stock. This step matters most in a tank that's already near its stocking capacity, since a disease outbreak in a crowded tank spreads faster and is harder to treat without additional stress to fish that have little spare bioload capacity to buffer treatment-related water changes. Skipping quarantine is a common cause of sudden, unexplained losses in otherwise healthy, well-stocked tanks.

Worked Example: Stocking a 20-Gallon Community Tank

Because bioload (the amount of waste a tank's filtration must process) depends on fish size, activity level, and waste production rather than length alone, a sensible stocking plan for a community tank typically mixes a modest number of small, low-waste schooling fish with one or two slightly larger, calmer species, leaving a safety margin below any rule-of-thumb maximum.

Worked example: a 20-gallon (roughly 75-litre) community tank stocked with 6 neon tetras, 5 cherry barbs, and a single bristlenose catfish sits comfortably below most stocking guidelines once the tank is fully cycled (see below) and filtered appropriately for that bioload, while still giving a visually full, active community. Adding a second bristlenose or a similarly sized bottom-dweller on top of this stocking would be pushing toward the upper end of what a 20-gallon tank's filtration can typically process, which is why experienced aquarists generally recommend stocking to roughly 80% of any calculated maximum rather than the absolute limit, leaving a buffer for fish growth and feeding fluctuations.

How Does Planted vs Unplanted Tank Stocking Differ?

A heavily planted aquarium can often support a somewhat higher fish stocking density than an identical unplanted tank, because live plants absorb some of the nitrogenous waste (particularly nitrate) that would otherwise need to be processed entirely by the filter's beneficial bacteria, and well-oxygenating plants also contribute to the dissolved oxygen levels referenced above. This doesn't mean stocking guidelines should simply be stretched proportionally for any planted tank — the actual benefit depends heavily on plant mass, species, and lighting quality, and a sparsely or poorly planted tank provides very little of this additional buffering capacity. As a practical approach, most experienced aquarists still stock a planted tank conservatively using the same core principles described above (bioload and filtration capacity first), treating a healthy plant mass as a welcome additional safety margin rather than as a reason to significantly increase the base stocking number.

How Long Should a New Tank Run Before Adding the Full Stocking Plan?

A newly set-up aquarium needs to complete the nitrogen cycle (covered above) before it can safely support its intended fish population, which typically takes around 4-6 weeks using a fishless cycling method, where ammonia is added deliberately to establish the beneficial bacteria colony before any fish are introduced at all. Attempting to stock a tank to its full calculated capacity immediately after setup, before this bacterial colony has established, is one of the most common causes of "new tank syndrome" — a spike in ammonia and nitrite that can be lethal to fish even in a tank that would be perfectly safely stocked once properly cycled. The practical approach most experienced aquarists recommend is to cycle the tank fully first, then add fish gradually in small batches over several more weeks even after cycling is complete, rather than introducing the full stocking plan all at once.