Try the tool above
Use the free tool at the top of this page first, then read the guide below to understand the result and use it well. Choosing fish without knowing how long they live leads to sad surprises, because a fish bought on impulse may need a decade of care. This estimator shows typical lifespan ranges and adult sizes for common aquarium species, so you can plan stocking with honest expectations instead of pet-shop optimism.
How to use it
Start by entering the numbers the tool asks for, using real measurements where you can, then press the calculate button and read the result box carefully. Check the result against common sense for your own setup, adjust one input at a time if the answer surprises you, and use the result as a daily plan rather than a one-off number.
Worked example
For example, a betta typically lives three to five years and reaches about six centimetres, while a goldfish can live ten to twenty years and reach twenty centimetres. The same tank cannot fairly hold both plans, and the estimator makes that difference obvious.
How the calculation works
The tool uses a simple, transparent rule based on the numbers you enter, so you can check the answer by hand if you ever doubt it. It does not guess, hide its working, or pretend to know more than the inputs allow, and that honesty is deliberate, because a pet care plan you can follow matters more than a complicated formula you cannot trust. If you change one input, such as weight, fish count, or tank size, the result moves in the direction you would expect, which is a good quick check that you typed the numbers correctly.
Why this matters
Lifespan matters for tank size too, since a small cute pleco can grow to thirty centimetres and outlive several other fish in the same tank. Knowing the adult size before you buy prevents overcrowding years later, when rehoming a large fish is hard.
Keep the routine going
The best plan is the one you actually repeat, so tie the result to something you already do every day, such as the morning feed, the evening tidy, or the weekly shop. Keep the tool result visible for the first two weeks, note any day you miss, and do not restart from scratch after a missed day, because one missed day matters far less than giving up the routine entirely and starting again next month.
Three tips that help
Buy fish in groups where the species needs company, because lonely schooling fish stress and live shorter lives than fish kept properly. Check adult size, not shop size, when you choose a tank, because many fish are sold as juveniles at a fraction of their final length. Keep water stable rather than chasing perfect numbers, because steady conditions add years while constant chemical swings shorten lives.
Common mistake to avoid
Do not mix very short-lived and very long-lived species without thinking, because their care needs and tank plans will pull in different directions over time.
Frequently asked question
How long do aquarium fish live? It varies hugely by species: guppies live about two to three years, neon tetras five to eight years, and goldfish ten to twenty years, as shown in this estimator.
Put the result into practice
Save or print your result, put it where the person who feeds or cares for your animals will see it, and review it whenever weight, fish count, or tank setup changes. Small steady routines beat perfect plans that nobody follows, so keep the plan simple, repeat it daily, and adjust slowly when the numbers tell you something has changed.