Aquarium Stocking Calculator

The answer depends on much more than just gallons. Fish size, waste output, your filtration system, plant density, and maintenance schedule all affect how many fish your tank can support. The old "one inch per gallon" rule is dangerously oversimplified — a 10-inch Oscar produces far more waste than 10 one-inch neon tetras. Use this calculator to get an accurate stocking score based on your exact setup.

Bioload is the amount of organic waste (ammonia, nitrite, nitrate) that the living creatures in your tank produce. High bioload means your filtration and beneficial bacteria must work harder to process waste. When bioload exceeds your system's capacity, ammonia and nitrate spike, stressing or killing fish. The stocking score in this calculator expresses your current stock as a percentage of your tank's estimated capacity.

Live plants absorb ammonia and nitrate directly, reducing the amount your filter needs to process. A densely planted tank can support significantly more bioload than a bare tank with identical filtration. This is why heavily planted Nature Aquarium tanks can appear overstocked but remain stable.

Yes — significantly. A canister filter provides more capacity than a hang-on-back filter, and a sump system more still. Conversely, no filtration at all drastically reduces your effective capacity. For saltwater tanks, running a protein skimmer alongside a sump provides the highest capacity. Always match your filtration to your stocking goals.

A 'fully stocked' tank (75–95% bioload) is running near capacity. It can be stable with excellent filtration and consistent maintenance, but there is little room for error — a missed water change or filter malfunction can quickly cause ammonia spikes. An 'overstocked' tank (above 95%) will typically show elevated nitrates, stressed fish, increased aggression, and reduced lifespan. We recommend keeping most community tanks in the 'balanced' range (55–75%) for a healthy margin.

Yes, but much less than fish. Invertebrates like cherry shrimp and nerite snails have much smaller bodies and produce far less ammonia per individual. A colony of 20 cherry shrimp typically contributes less bioload than a single adult betta.

The one inch per gallon rule is widely regarded as one of the most problematic guidelines in the fishkeeping hobby. It ignores body mass (a 3-inch goldfish has far more mass than a 3-inch kuhli loach), waste production rates, filtration, water volume displacement, and swimming space. A goldfish, for example, may require 20+ gallons for a single fish despite being 5–6 inches. Always use a bioload-based calculation rather than length alone.

The recommended water change frequency and volume depends on your bioload score. For a lightly stocked freshwater tank, 20–25% weekly is typically sufficient. At full capacity, you may need 30–40% weekly to keep nitrates in check. For saltwater tanks, more conservative water changes of 10–15% weekly are standard even at light stocking, since reef ecosystems are more sensitive to parameter swings. This calculator provides a personalized recommendation based on your current stocking score.

Different species thrive at different water temperatures. When you add multiple species to this calculator, it automatically checks whether all species have an overlapping temperature range. If species A is comfortable at 72–78°F and species B needs 78–84°F, the only compatible temperature is exactly 78°F — which is stressful for both. The calculator will flag incompatible pairs so you can adjust your stocking plan before you buy.

Yes. You can save any stocking plan using the 'Save Plan' button that appears when you have results. Registered members can name their plans and access them later from the 'My Plans' tab. You can also share any plan via a link. Create a free Shoal & Stem account to keep your plans permanently.