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Aquarium Stocking Calculator

Calculate safe fish stocking levels

Max Fish Inches

29 inches

Small

19

Filter

145 GPH

Maximum Fish Inches

29

total inches of fish

Small Fish

19

(1-2")

Medium Fish

8

(3-4")

Large Fish

5

(5-6")

Recommended Filter145 GPH

Bioload Assessment

Moderate - Weekly maintenance

Stocking Tips

Stock Gradually

Add fish slowly over weeks to let your filter bacteria adjust.

Consider Adult Size

Always calculate based on adult fish size, not current size.

What You'll Need

Fluval 307 Performance Canister Filter

Fluval 307 Performance Canister Filter

$120-$1604.6
View on Amazon
Saltwater Master Test Kit 550-Test with Sea Salt

Saltwater Master Test Kit 550-Test with Sea Salt

$48-$554.5
View on Amazon
HITOP 100W Adjustable Aquarium Heater Submersible

HITOP 100W Adjustable Aquarium Heater Submersible

$12-$184.3
View on Amazon
Fluval 307 Performance Canister Filter

Fluval 307 Performance Canister Filter

$120-$1604.6
View on Amazon
Saltwater Master Test Kit 550-Test with Sea Salt

Saltwater Master Test Kit 550-Test with Sea Salt

$48-$554.5
View on Amazon
HITOP 100W Adjustable Aquarium Heater Submersible

HITOP 100W Adjustable Aquarium Heater Submersible

$12-$184.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Example Calculations

129-Gallon Community Tank

Inputs

Tank Size29 gallons
Fish TypeCommunity Tropical

Result

Max Total Inches29 inches
Small Fish (1-2 in)19 fish
Medium Fish (3-4 in)8 fish
Large Fish (5-6 in)5 fish
Filter Needed145 GPH

Max inches = 29 x 1.0 = 29. Small fish = floor(29 / 1.5) = 19. Medium = floor(29 / 3.5) = 8. Large = floor(29 / 5.5) = 5. Filter = 29 x 5 = 145 GPH. Bioload: Moderate.

255-Gallon Cichlid Tank

Inputs

Tank Size55 gallons
Fish TypeCichlids

Result

Max Total Inches28 inches
Small Fish (1-2 in)18 fish
Medium Fish (3-4 in)7 fish
Large Fish (5-6 in)5 fish
Filter Needed440 GPH

Max inches = 55 x 0.5 = 27.5, rounded to 28. Small fish = floor(27.5 / 1.5) = 18. Medium = floor(27.5 / 3.5) = 7. Large = floor(27.5 / 5.5) = 5. Filter = 55 x 8 = 440 GPH. Bioload: High.

Frequently Asked Questions

Q

What is the inch per gallon rule?

The classic rule is 1 inch of fish per gallon of water. However, this is a simplification - adult fish size, body shape, and bioload all affect true stocking capacity.

  • Slim-bodied fish (neon tetras, danios): 1 inch per gallon works well as a guideline
  • Heavy-bodied fish (goldfish, oscars): use 0.3–0.5 inches per gallon due to higher bioload
  • Always use adult fish size for calculations – a 1-inch baby oscar grows to 12–14 inches
  • Surface area matters more than volume: long/wide tanks support more fish than tall/narrow ones
  • Live plants absorb nitrates and can safely increase stocking by 10–20% over the base rule
Q

Why can I keep fewer goldfish than tropical fish?

Goldfish produce much more waste than tropical fish of similar size. They need 10+ gallons per fish minimum, while many tropical fish can thrive in smaller spaces.

  • Goldfish produce 2–3× more ammonia than tropical fish of the same body mass
  • Fancy goldfish: minimum 20 gallons for the first fish, +10 gallons per additional fish
  • Common/comet goldfish grow to 10–12 inches and need 30–55+ gallons each (or a pond)
  • Tropical community fish (tetras, rasboras): 5–10 gallons for a school of 6
  • Goldfish need cold water (65–75°F) and cannot be mixed with tropical species (75–82°F)
Fish TypeMin. Tank SizeInches/Gallon Rule
Community Tropical10 gal1.0 in/gal
Cichlids30 gal0.5 in/gal
Fancy Goldfish20 gal0.3 in/gal
Common Goldfish55 gal0.3 in/gal
Q

What filter size do I need?

Your filter should turn over the tank volume 4-5 times per hour for tropical fish, 8-10 times per hour for goldfish or messy fish like cichlids.

  • Community tropical tank (29 gal): filter rated 145–175 GPH (gallons per hour)
  • Cichlid tank (55 gal): filter rated 440–550 GPH for heavy bioload
  • Goldfish tank (40 gal): filter rated 320–400 GPH due to high waste production
  • Canister filters provide the best filtration for tanks over 40 gallons ($80–$200)
  • Hang-on-back (HOB) filters are more affordable ($20–$50) and sufficient for tanks under 40 gallons
Q

How long should I wait before adding fish to a new tank?

Cycle your tank for 4-6 weeks before adding fish so beneficial bacteria can establish. Then add only a few fish at a time over several weeks.

  • Fishless cycling with pure ammonia takes 4–6 weeks to establish beneficial bacteria
  • Test water parameters: ammonia and nitrite should read 0 ppm before adding any fish
  • Add 2–3 hardy fish first (danios, white clouds) and wait 2 weeks before adding more
  • Using established filter media or bottled bacteria can speed cycling to 1–2 weeks
  • Perform 25% water changes weekly during cycling to keep ammonia below 4 ppm

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Last Updated: Mar 26, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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