How Many Fish Can I Put in My Tank?
The old "one inch of fish per gallon" rule is a rough starting point, not a law. It ignores fish body mass, waste output (bioload), swimming style, and territory needs.
A better approach: start from the tank footprint and filtration, target 75-80% of the theoretical maximum to leave a safety margin, and always cycle the tank before adding livestock.
Heavy waste producers like goldfish and large cichlids need far more space per inch than slim tetras. Use the calculator on this site to estimate a safe stocking level for your specific tank.