Last updated: 2026-06-22

The inch-per-gallon rule

The inch-per-gallon rule is the most widely cited starting point for aquarium stocking. It states that a tank can comfortably support 1 inch of fish body length for every US gallon of water. A 20-gallon tank therefore supports up to 20 inches of fish total — ten 2-inch tetras, four 5-inch cichlids, and so on.

The calculation is straightforward:

The default factor is 1. You can reduce it (try 0.5) for species with high bioload or poor water-quality tolerance, or increase it slightly (up to 2) for very small, slender species in a heavily filtered setup.

Worked example

A 20-gallon (75.7 l) freshwater community tank stocked with neon tetras (adult length ≈ 2 inches / 5 cm):

• Stocking capacity: 20 × 1 = 20 inches (50.8 cm)
• Fish count: 20 ÷ 2 = 10 tetras

As a rough estimate, this is a reasonable starting point. In practice, neon tetras have low bioload and a 20-gallon tank with good filtration could comfortably hold 12–15 of them.

Limitations

This rule is a rough planning heuristic, not a guarantee of water quality. It ignores:

• Bioload: a 6-inch goldfish produces far more ammonia than six 1-inch fish.
• Body shape: a deep-bodied discus or oscar has much greater mass than a slender pencilfish of the same length.
• Surface area: oxygenation depends on the water-to-air surface area, not just volume.
• Filtration capacity: a well-designed filter allows higher stocking than an undersized one.
• Species compatibility: aggression, territory, and swimming levels are not captured by length.

For saltwater tanks, the guideline is typically much stricter — around 1 inch per 5 gallons (0.2 factor). Reef aquariums in particular are sensitive to bioload and often stocked even more conservatively.

Always introduce fish gradually and monitor ammonia, nitrite, and nitrate levels. For an accurate tank volume calculation, see Aquarium Volume Calculator.