Aquarium stocking calculator

Every fish contributes its adult length multiplied by how many you keep, with an extra weighting that grows for fish over 10cm. That weighting matters: a single 30cm fish produces far more waste than five 6cm ones, even though the lengths add up the same. The inch-per-gallon rule misses this entirely, which is why it fails so badly on large species.

That fish total is then compared against the tank’s capacity, which starts from your water volume and is adjusted three ways:

• Water type. Marine tanks are held to roughly half the stocking of freshwater and brackish to three-quarters, reflecting how much less forgiving they are of a swing in water quality.
• Filter turnover. Capacity scales with how your turnover compares to the target for your water type — four times the tank volume per hour for freshwater, five for brackish, six for saltwater. The adjustment is capped between 0.65x and 1.6x, because an enormous filter does not make a small tank limitless. Protein skimmers and UV clarifiers are deliberately excluded: they treat water, but they are not turnover.
• Live plants. In freshwater only, each plant adds one percent of capacity up to a maximum of ten percent, reflecting the waste nitrogen they take up.

Fish marked as juveniles or fry are still planned at their adult size, because they will get there. Where that applies, the result also shows what the tank is carrying today, so you can see how much headroom you are actually working with.

Adult sizes come from the Studio Scaped fish knowledge base. Where we have no size on file for a species, the calculator says so and asks you for the number rather than guessing one.

Most stocking calculators only do freshwater. This one holds a marine tank to roughly half the stocking of the same volume of freshwater, and a brackish tank to three-quarters — not because marine fish are dirtier, but because the systems are far less forgiving of a swing in water quality, and because the target turnover is higher: six times the tank volume per hour for saltwater, five for brackish, four for freshwater.

Protein skimmers and UV clarifiers are deliberately left out of the turnover sum. They treat water, but a skimmer’s air-and-water rating is not aquarium turnover, and counting it would quietly tell you that you can stock more fish than you can. Add one and you will see it listed with the reason it does not move the number.

Plants only earn a capacity allowance in freshwater, where they take up waste nitrogen — one percent each, capped at ten percent no matter how heavily planted the tank is.

The long-standing tool most people reach for is AqAdvisor, and it is freshwater-only. If you keep a marine or brackish tank, this calculator covers water types it does not. It also works natively in litres as well as gallons, which matters if you are outside the US and tired of converting your tank size before you can use a tool.

The bigger difference is what happens after the estimate. Most calculators give you a percentage and stop. This one hands the whole setup — fish, plants, filtration, volume — straight into the free Studio Scaped tracker, where the same figure keeps updating as your fish grow, alongside your water tests and maintenance history. The arithmetic is not an approximation of the tracker’s; it is literally the same code.

Adult sizes come from a knowledge base of more than 1,200 species rather than a short built-in list, and where we genuinely have no size on file the calculator says so and asks you, rather than quietly guessing a number that changes your answer.