An aquarium stocking calculator helps you plan a healthy fish tank by checking species compatibility, calculating bioload, and recommending water change frequency. Overstocking is the most common mistake in fishkeeping — it leads to poor water quality, stressed fish, and disease outbreaks. Enter your tank size and add fish below to see if your setup is balanced.
Tank Setup
How to Use the Aquarium Stocking Calculator
Overstocking is the leading cause of poor water quality, fish stress, and disease in home aquariums. Every fish produces ammonia through waste and respiration, and your tank's biological filter can only process so much. An aquarium stocking calculator helps you plan a balanced community by estimating the bioload each species places on your system, checking temperament and temperature compatibility, and recommending a water change schedule that keeps nitrate levels safe.
Step 1: Enter Your Tank Size
Start by entering your aquarium size in gallons. This is the total water volume, not the physical dimensions. Remember that decorations, substrate, and equipment reduce actual water volume by roughly 10-15%, so a 20-gallon tank may hold only 17-18 gallons of water. If you are unsure of your tank size, measure the interior dimensions in inches and multiply length x width x height, then divide by 231 to get gallons.
Step 2: Add Fish Species
Select a species from the dropdown and click Add. You can adjust the quantity using the plus and minus buttons next to each fish in the list. The calculator includes popular freshwater species with their bioload requirements: Neon Tetras (1 gallon each), Guppies (2 gallons each), Corydoras (2 gallons each), Bettas (2.5 gallons minimum), Mollies (3 gallons each), Dwarf Gouramis (5 gallons each), Angelfish (10 gallons each), and Goldfish (20 gallons each for proper care).
Step 3: Check the Bioload Bar
The stocking level bar shows what percentage of your tank's capacity is used. Green (under 75%) means your tank is comfortably stocked with room for error. Yellow (75-100%) means moderately stocked and requires diligent maintenance. Red (over 100%) means overstocked — you need a larger tank or fewer fish. Aim to stay under 75% for the healthiest environment, especially if you are a beginner.
Step 4: Review Compatibility Warnings
The calculator checks for common compatibility issues: aggressive fish paired with peaceful species, schooling fish kept below their minimum group size, mixing cold water fish (like Goldfish) with tropical species, and Bettas housed with other surface dwellers. Each warning explains the specific problem and suggests a solution, helping you avoid costly and stressful mistakes before you buy.
Step 5: Follow Water Change Recommendations
Based on your stocking level, the calculator recommends a water change frequency. Lightly stocked tanks can get by with biweekly partial changes, while heavily stocked tanks need weekly or even twice-weekly maintenance. Consistent water changes are the single most important thing you can do for your fish's health, diluting nitrates and replenishing trace minerals that the biological filter cannot restore.
General Stocking Guidelines
The old "one inch of fish per gallon" rule is outdated and unreliable. A 10-inch Oscar produces far more waste than ten 1-inch Neon Tetras. This calculator uses species-specific gallon requirements based on adult size, waste production, and activity level. Always research a species before purchasing, use a quality filter rated for your tank size or larger, cycle your tank before adding fish, and test water parameters regularly with a liquid test kit.
Frequently Asked Questions
Is this aquarium stocking calculator free?
Yes, this aquarium stocking calculator is completely free with no signup or account needed. All calculations run locally in your browser, so your fish list and tank data are never sent to any server.
Is my data private?
Absolutely. Everything runs in your browser using JavaScript. Your tank size, fish selections, and stocking data never leave your device. No cookies, no tracking, no server-side storage.
How does the bioload percentage work?
Bioload represents the biological load your fish place on the tank's filtration system. Each fish species needs a certain number of gallons. The calculator sums the gallon requirements for all your fish and compares it to your tank size. Under 75% is ideal, 75-100% is moderate, and over 100% means your tank is overstocked.
Can I keep a Betta with other fish?
Bettas are classified as aggressive and are best kept alone or with very peaceful bottom-dwelling species like Corydoras or Cherry Shrimp. This calculator will flag compatibility warnings if you add a Betta alongside other surface or mid-level swimmers that may trigger territorial behavior.
What does schooling fish mean?
Schooling fish like Neon Tetras and Corydoras need to be kept in groups of a minimum number (usually 4-6) to feel safe and display natural behavior. Keeping fewer than the minimum can cause stress and health problems. The calculator warns you if you add fewer than the recommended school size.
How often should I change the water in my aquarium?
Water change frequency depends on your stocking level. Lightly stocked tanks (under 50%) can do 15-20% changes every two weeks. Moderately stocked tanks (50-75%) should do 20-25% weekly. Heavily stocked tanks (75-100%) need 25-30% weekly changes. Overstocked tanks need even more frequent changes.
Can I mix tropical and cold water fish?
No, tropical and cold water fish have different temperature requirements and should not be kept together. Goldfish, for example, prefer 65-72F while most tropical species need 74-82F. This calculator checks temperature compatibility and warns you if you mix incompatible temperature groups.
What tank size do I need for beginners?
A 20-gallon tank is ideal for beginners. Larger tanks are actually easier to maintain because they are more forgiving of water quality fluctuations. Start with hardy, peaceful species like Neon Tetras, Guppies, or Corydoras. Use this calculator to plan a balanced community before purchasing any fish.