This free aquarium quarantine tank planner tells you what size quarantine tank you need, what heater and filter flow it needs, and how many days new fish should stay in observation. It runs fully in your browser, so nothing you type is sent to a server, and you need no account and no sign-up to use it.
Aquarium problems usually build slowly. A tank that looks fine today can drift over weeks when feeding, stocking, or water changes stay unmeasured. Small numbers, checked early, stop most trouble before fish show stress. This tool turns a guess into a number you can act on, and that number is easier to trust than a rough rule of thumb from a shop assistant.
To use the tool, type in Number of new fish, Average fish length (cm), Main tank size (litres). Press the calculate button and the tool shows the result at once. If you change any number, press calculate again to see the new result right away. Try a few values to see how sensitive the result is to each input.
Example: five new fish with an average length of 5 cm need a quarantine tank of about 50 litres, a 50 watt heater, and a filter moving about 250 litres per hour. Keep them under observation for 21 days before they join the main tank.
Set the quarantine tank up before you buy fish. A cycled filter and stable temperature matter far more than a fancy tank, so run the spare filter in the main tank for two weeks first if you can.
Watch feeding and droppings closely during observation. Fish that eat well and produce normal waste for three weeks are far safer to add than fish that only looked healthy in the shop bag.
Do small, frequent water changes in quarantine. A small tank fouls faster than a big one, so change about a quarter of the water every two or three days and test for ammonia weekly.
Common question: Why 21 days of observation? Because most common fish diseases show within three weeks. A shorter stay can miss a slow infection, and one sick fish can infect a whole established tank.
Common question: Do I need a filter in quarantine? Yes. Even a simple sponge filter gives bacteria a home and keeps ammonia down. Without one, waste builds up fast in a small tank and stresses fish that are already stressed.
Common question: Can I use water from the main tank? Yes, it helps match temperature and chemistry, but it does not replace a cycled filter. Use main-tank water for filling, and still run a mature sponge filter in the quarantine tank.
Keep a short notebook by the tank. Write the date, the numbers you entered, the result, and anything you changed afterwards. That short record turns a vague feeling that something changed into clear evidence. After a month you can look back and see which choices helped and which ones did not, and that makes the next decision much easier.
The most common mistake is changing too much at once. When several things change in the same week, you cannot tell which change caused the result you see. Change one thing, wait, watch the fish and the water, and only then change the next thing. Slow, single changes keep a tank stable, and stable tanks keep fish healthy.
Recheck the numbers whenever anything changes in the tank. New fish, a new filter, a different food, or a change in room temperature can all shift the right answer. A quick recheck takes less than a minute and stops an old number from quietly becoming wrong. Bookmark this page so the recheck is easy.
This tool is free to use as often as you like. Bookmark it, recheck the numbers when your tank changes, and keep the daily routine steady. Small, steady habits matter far more for an aquarium than any single perfect number, and a calm routine is what fish respond to best. If a number surprises you, run the tool twice with different inputs before you act on it, and trust the steadier result.