Fish Density Calculator

The general rule for freshwater tropical fish is 1 inch of fish per gallon of water. For a 20-gallon tank, you could have about 20 inches of total fish (e.g., 10 two-inch fish). However, this varies by fish type: goldfish need 2 gallons per inch, saltwater fish need 5 gallons per inch, and koi need 10 gallons per inch due to their size and waste production.

The inch per gallon rule is a simple guideline stating that you need 1 gallon of water for each inch of fish (measured from nose to tail). While useful as a starting point, it has limitations - it works best for small, slim fish and doesn't account for body mass, swimming patterns, or waste production of larger or rounder fish.

Goldfish produce significantly more waste than tropical fish of similar size, requiring extra filtration capacity and water volume to maintain water quality. Fancy goldfish can grow to 6-8 inches and need at least 20 gallons for the first fish plus 10 gallons for each additional fish. Common goldfish grow even larger and are best suited for ponds.

Saltwater fish require approximately 5 gallons per inch of fish - much more than freshwater. This accounts for their territorial nature, sensitivity to water quality changes, and the need for stable water parameters. A 50-gallon saltwater tank might only support 10 inches of total fish comfortably.

Always calculate based on adult size! Fish grow, and planning for their full size prevents overcrowding problems later. A tank that seems spacious with juvenile fish can become severely overstocked within months. Research the adult size of each species before purchasing.

Overstocking leads to: poor water quality from excess waste, low oxygen levels, increased disease, stress and aggression, stunted growth, and shortened lifespan. Signs include fish gasping at the surface, frequent illness, cloudy water, and aggressive behavior. If overstocked, increase water changes, add filtration, or rehome some fish.