Photo: Kamscharmog · CC BY-SA 3.0
husbandry · freshwater
How to quarantine new fish
A bare-bottom 10–20 gallon with a seeded sponge, heater, and lid. Observe two to four weeks, acclimate properly, and treat only when you see a problem, not a medication cocktail on day one.
Published 2026-09-09
Minimum viable quarantine
Quarantine is a second tank where new fish live long enough to show problems before they share water with your display. You do not need a fortress. You need a cycled biological filter, stable temperature, a lid, and eyes on the fish for two to four weeks.
A 10–20 gallon bare-bottom tank is enough for most freshwater community purchases. Bare bottom makes waste visible and easy to siphon. Add a PVC elbow or a plain cave for cover, a heater, and a lid, jumpers and splash happen in QT too. Dedicated net and siphon so you are not ferrying pathogens back to the display on shared gear.
Seeded sponge first
An uncycled quarantine tank that looks empty still produces ammonia. The reliable fix is a sponge filter that has been running on the display for weeks. Move that seeded sponge (or a spare sponge you keep on the display outflow) into the QT before the fish arrive. Squeeze it only in old tank water when you clean it.
Test ammonia and nitrite in the QT the same way you would during a cycle. If numbers climb, do partial water changes and confirm the sponge is still seeded, do not 'wait it out' with livestock in the box the way you might wait on a fishless cycle. Link this to how you already cycle and water-change the display; QT is not a license to skip chemistry.
Acclimation: float, drip, net
Float the sealed bag in the QT for 15–20 minutes so temperatures match. If pH, hardness, or salinity differ much from the shop bag, drip QT water into a container with the fish for 30–60 minutes so the change is gradual. Then net the fish into the quarantine tank and dump the bag water down the drain, never into the QT or the display.
Lights low, cover available, no tank mates on day one. Feed lightly once they settle. Match QT parameters toward the display over the last few days before the move so the final transfer is not a second shock.
Watch, then treat
Observe for clamped fins, flashing, spots, fungus-looking patches, rapid breathing, or refused food. Ich and other common issues get their own protocols, see how to treat ich when salt-grain spots show up. Treat only when you can name what you are treating. Remove carbon if you dose a medication, follow one bottle, and finish the course.
Species that often arrive stressed from shops, including honey gouramis mislabeled next to dwarfs, neons, and bettas, still benefit from the same QT habit. Skipping quarantine for a single fish into an empty tank is a calculated risk. Skipping it into an established community is how one bag resets months of stable water. When the observation window is clean and parameters match, net the fish into the display and leave the QT ready for the next purchase.
Gear for a working quarantine tank
A few listings we would open for this job. Skip anything you already own that still works.
hygger sponge filter, small
Move a seeded sponge from the display tank into the QT so you are not starting ammonia day one. Gentle flow, fry-safe intake, and easy to squeeze in old tank water.
Air pump and tubing sold separately. If the display already runs a spare sponge, that is your QT biofilter. Never rinse it under the tap.
See this listingAqueon Adjustable Pro heater, 50W
Quarantine water should match the display temperature. A reliable adjustable heater on a 10-gallon QT holds the number so stress is not compounded by chill.
Use 100W on a 20-gallon QT. Always confirm with a separate thermometer. Skip a second heater if one unit already holds the target.
See this listingAPI Freshwater Master Test Kit
An uncycled QT that looks clear can still cook fish with ammonia. Liquid reagents for ammonia, nitrite, nitrate, and pH tell you whether the seeded sponge is actually working.
Freshwater only. Skip strips as your only tool. If the bottles you already own are in date, you do not need another kit.
See this listing
Frequently asked questions
How long should I quarantine new fish?+
Two to four weeks of observation in a separate tank is the usual range. Shorter windows miss slow-building problems. Longer is fine if you have the space and the fish are eating and looking normal.
Do I medicate every new fish on arrival?+
No. Watch first. Treat when you see a clear problem and can name it. Blind prophylactic cocktails stress fish and wreck biofilters.
Can I skip quarantine if I only bought one betta?+
You can. It is a gamble, not a moral failure. A single cup fish into an empty display skips the hitchhiker risk to other livestock, but the display then becomes the hospital if something shows up. A cheap 10-gallon QT is still the safer habit once you keep a community.
Should I pour the bag water into the quarantine tank?+
No. Net the fish, dump the bag water. Shop water can carry ammonia, meds, and hitchhikers. Temperature-match and drip-acclimate, then transfer the fish only.