What is the footprint method for aquarium stocking?

Instead of volume, footprint (length × width) is used as the basis. Small fish up to 2 inches need about 15 cm² per cm of body length, medium fish 2–6 inches around 25 cm², and large fish over 6 inches at least 40 cm². Wide, shallow tanks perform better than tall, narrow columns.

Why is overstocking a problem in aquariums?

Too many fish produce more ammonia than the filter can break down. NO₂ and NO₃ rise, O₂ drops, and stress increases susceptibility to disease. The result is poor water quality, frequent losses and demanding maintenance.

Which volume do I use for stocking density?

For the outdated 1-inch-per-gallon rule of thumb, use net volume (after subtracting substrate and decor). For the more modern footprint method, tank length and width are enough – height and volume play a minor role.

Fish Stocking Calculator – Footprint Method & Compatibility

Q: Is the 1-inch-of-fish-per-gallon rule still accurate?

No. The rule dates back to the 1950s and ignores body shape, bioload and swimming space. A 4-inch discus and a 4-inch neon tetra create completely different loads. The footprint method (tank floor surface area) is more realistic.

Q: Can different fish species be kept together?

Not automatically. Parameters like pH, temperature, GH and KH need to match for all species. Territorial behavior, predation and fin-nipping tendencies also matter. In Aqualizer, the compatibility database checks exactly that for your species combination.

Your stocking

Tank

Length (cm)

Width (cm)

Net vol. (L) for the old rule of thumb

Fish & species

Species (optional) Length (cm) Count

Stocking density

Enter tank size and at least one species.

Why is the 1-inch rule outdated – and what's better?

The problem with 1 inch per gallon: the rule treats a 4-inch discus (deep body, high bioload, needs swimming room) the same as 4-inch neon tetras (slim body, minimal bioload). It also favors tall, narrow column tanks over wide, shallow tanks – even though the latter are better for almost all fish.

The footprint method uses length × width (cm²) as its basis. Every cm of fish body length needs a different amount of footprint depending on body size:

Fish length	Footprint / cm	Example
up to 5 cm	15 cm²/cm	Neon tetra (4 cm) → 60 cm²
5–15 cm	25 cm²/cm	Guppy (5 cm) → 125 cm²
over 15 cm	40 cm²/cm	Discus (20 cm) → 800 cm²

Note: even the footprint method is only a guideline. Territorial behavior, filtration, planting and species-specific needs can shift the value up or down. In Aqualizer this is checked per species with real database values.

What this check can't do

Do the species actually get along?

Neither the 1-inch rule nor the footprint method checks whether your guppy and discus need the same pH, whether your barb is a fin-nipper, or whether the catfish hunts your shrimp at night. That takes a real database.

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Compatibility check

Predation, territorial behavior, fin-nipping – over 10,000 species combinations in the database, checked instantly whenever you add a new species.

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Water parameter match

Does your tank's pH suit the new species? Are temperature, GH and KH within tolerance? Everything compared automatically.

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Stock overview

All the animals in your tank at a glance – count, species, date. Log losses and offspring, with full history included.

More tools

Find the right stocking

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Species database

Over 2,000 species with pH, temperature, GH, KH, minimum tank size and social behavior – linked to the in-app stocking planner.

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Track water parameters

Log pH, GH, KH, NO₂, NO₃ after every water change – and instantly see whether the values suit your stocking.

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Disease treatment

Suspect an illness? Enter symptoms, get a treatment plan, and follow it step by step, day by day.

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Wishlist

Bookmark future species. The compatibility check runs automatically against your current stock – before you buy.

Frequently asked questions

Is the 1-inch-of-fish-per-gallon rule still accurate?

No. The rule dates back to the 1950s–70s and ignores body shape, bioload and swimming space. A deep-bodied discus and a slim neon tetra aren't comparable at the same length. The footprint method (cm² of tank floor per cm of fish length) is more realistic because it favors wide, shallow tanks – matching real-world practice.

How many fish fit in my tank using the footprint method?

Small fish (under 5 cm) need about 15 cm² per cm of body length, medium fish (5–15 cm) about 25 cm². Example: a 100 × 40 cm tank = 4,000 cm². With small fish (4 cm, 60 cm² needed each), that's theoretically about 66 fish. Recommendation: use 60–80% of capacity, never 100%.

Can different fish species be kept together?

Not automatically. pH, temperature, GH and KH need to match for all species. Territorial behavior, predation and fin-nipping tendencies also matter. In Aqualizer, the compatibility database checks exactly that for your species combination – with a warning before you buy.

What happens with overstocking?

The filter can't break down the ammonia load. NO₂ rises, O₂ drops, and fish become stressed and prone to disease. Regular water changes can help temporarily, but are no permanent substitute for appropriate stocking.

Do shrimp and snails count toward stocking?

For stocking density calculations: barely. The bioload of shrimp and snails is so low that 20 shrimp in a 15-gallon tank is not a problem. Snails often self-regulate based on available food.

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Choose the right stocking

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Fish Stocking Calculatorfor Aquariums