What Is Aquarium Stocking?
Aquarium stocking is how many fish your tank can safely hold without polluting the water or stressing the inhabitants. Overstocking is the #1 cause of ammonia spikes, disease, and fish loss for beginners. A stocking calculator helps you plan before you buy.
The One-Inch-Per-Gallon Rule
The classic guideline says you can keep one inch of adult fish per gallon of water. A 20-gallon tank, for example, supports about 20 inches of fish total. We apply a 20% buffer for gravel, decorations, and water displacement, so usable volume is roughly 80% of the tank's dimensions.
How to Use This Calculator
1. Enter your tank's length, width, and height. 2. Pick inches or centimeters. 3. Select your fish type (small community, schooling, medium, or large cichlid). 4. See tank volume, stockable inches, and estimated fish count.
Stocking Reference
| Fish Type | Adult Size per Fish |
|---|---|
| Small community (neon tetra, guppy) | ~1 inch |
| Schooling (rasbora, small danio) | ~1 inch |
| Medium community (angel, platy) | ~2 inches |
| Large cichlid (oscar, jack dempsey) | ~4 inches |
Beyond the Rule of Thumb
- Filtration matters: Over-filter rather than under-filter.
- Temperament matters: Aggressive species need more space.
- Shape matters: Tall tanks hold less swimming room than long ones.
- Cycle first: Never stock a brand-new tank all at once.
This free tool gives a safe starting estimate. Research each species' adult size and behavior before committing to a final stock list.