The short answer
To quarantine a sick fish, move it to a separate, simple, cycled tank — often called a hospital tank — where you can observe it closely and treat it without affecting your main community or your plants and filter. Keep it bare and easy to clean, match the water to your main tank, and check water parameters first before considering any medication.
Setting up the quarantine tank
A quarantine tank doesn’t need to be fancy. A modest bare-bottom tank with a heater, a gentle sponge filter and a hiding spot is ideal. Bare glass makes it easy to spot waste and clean, and a sponge filter is safe if you later need to medicate.
The key is that the filter must be biologically active — running a spare sponge in your main tank’s filter for a few weeks keeps live bacteria ready. Match the temperature and chemistry to the fish’s home tank so the move itself doesn’t add stress. For a full walkthrough, see how do I set up a hospital tank?
Using it safely
Once the fish is settled, watch and test before you treat. Test the water, note the symptoms, and identify the likely problem — many issues improve with clean, warm, stable water alone. If medication is genuinely needed, research the right product for the condition and your species, and ask a vet or experienced fishkeeper before dosing. Do smaller, frequent water changes to keep the small volume clean.
This is general guidance for setting up quarantine, not a diagnosis or treatment plan.
Returning the fish
Only move the fish back once it has been eating, active and symptom-free for several days. Do it with the same slow acclimatisation. To prevent problems in the first place, quarantine new fish too — see do I need to quarantine new fish?