Aquarium Stocking & Equipment Calculator

Why the “1 Inch Per Gallon” Rule is a Myth

The old rule treats a 6-inch angelfish and six 1-inch neon tetras as equals, and it ignores everything that actually pollutes water: body mass, metabolism, and waste output. A single fancy goldfish produces more ammonia than twenty small tetras. Modern stocking uses a bioload approach — weight each fish by adult size and messiness, then compare the total against your real water volume. It is still a simplification (behavior, territory, and water parameters matter too), but it beats linear inches every time. And it always starts with the true volume, not the label.

How to Calculate True Aquarium Water Volume

Start with the glass: length × width × height ÷ 231 gives gallons (231 cubic inches per gallon). Then subtract displacement. Your substrate layer removes length × width × substrate depth ÷ 231 gallons. Rocks and driftwood are irregular, so estimate them as a percentage — about 15% of what is left is typical for a decorated tank, 5% for a sparse one, up to 25% for a heavily aquascaped build. The remainder is the number you should stock, heat, filter, and dose for. A labeled 20-gallon commonly lands at 16–17 gallons after this math.

Understanding Filter GPH and Turnover Rates

GPH — gallons per hour — tells you how much water a filter moves. Divide it by your actual tank volume and you get turnover. Aim for roughly 4× for light stock (shrimp, bettas, nano fish), 6× for typical communities, and 8–10× for messy animals like goldfish, cichlids, and turtles. Two caveats: manufacturer ratings assume clean media and zero lift height, so real flow runs lower, and some species (bettas, long-finned fish) dislike strong currents — baffle the outflow rather than under-filtering. Media volume matters as much as flow, since that is where your bacteria live.

The Nitrogen Cycle: A Beginner’s Guide to Cycling a Tank

Set up the tank, run the filter and heater, and introduce an ammonia source — fish food, pure ammonia, or a bottled starter. Within a week or two, bacteria begin converting ammonia to nitrite; a second wave of bacteria then handles nitrite, converting it to nitrate. You are “cycled” when your test kit shows 0 ammonia, 0 nitrite, and measurable nitrate within 24 hours of dosing — usually 4 to 6 weeks in. Then add fish slowly, a few at a time, so the bacteria colony can grow to match the new load. Skipping or rushing this step is the most common cause of dead fish in new tanks.

How to Choose the Right Heater Wattage for Your Room

Heater sizing is about the temperature gap, not just tank size. Find the difference between your target water temperature and the coolest your room gets. Small gaps (up to 5°F) need about 2 watts per gallon; moderate gaps (up to 10°F) about 3 watts per gallon; big gaps about 5 watts per gallon. Apply that to your actual water volume and round up to the nearest standard size — 25, 50, 100, 150, 200, or 300 W. For tanks above 55 gallons in cold rooms, two smaller heaters beat one big one: redundancy if one fails, and gentler, more even heating.