Do I need a quarantine tank?

This is the pick that makes a quarantine tank practical rather than theoretical, and the reason is the thing nobody tells beginners: a sponge filter can be cycled in about thirty seconds.

Keep a spare sponge sitting in your main tank or inside your main filter. It colonizes with the same bacteria your display tank runs on. When you need a quarantine tank, move that sponge across and the new tank has a working biological filter immediately, rather than the four to six weeks a bare new filter would take.

Two more things suit it to this job specifically. It holds no activated carbon, so there is nothing to strip out if a treatment is ever needed. And it has no impeller and no intake slot, which matters when the occupant is a small, stressed, possibly unwell fish.

The honest costs. Nothing comes in the box but the sponge: no pump, no airline, no check valve, and you need all three before it does anything. The pump is the noisiest object in the setup. It is visible in the tank, which does not matter here. And the squeeze trick only works if you have actually kept a spare sponge in the main tank — started from dry, it is as sterile as any other new filter.

A quarantine tank usually lives somewhere you did not plan for a tank — a spare room, a shelf, a corner of an office — and the pump is the part that decides whether you tolerate it being there.

One outlet drives one sponge filter comfortably, which is the entire requirement here.

The honest costs. Tetra sells the line by tank-size range and publishes no airflow figure we could find, so you cannot compare it against a rival on paper. One outlet, so running a second quarantine tank means a second pump or a gang valve. And no diaphragm pump is silent — the surface it stands on matters more than the brand, and a folded towel underneath is the cheapest fix there is.

When a pump stops, the airline becomes a siphon: water runs back down the tube, out through the dead pump and onto whatever the tank is standing on. A check valve fitted above the water line makes that impossible.

It matters more here than on a display tank, because a quarantine tank is usually the one sitting on furniture that was never meant to hold water.

The honest costs. Check valves are directional, and fitted backwards they block the air completely, which looks exactly like a dead pump. They stiffen and clog over a year or two, so treat them as a consumable rather than a fitting. And a valve is not a substitute for routing the airline above the water line in the first place.

The right heater for a quarantine tank is the cheap one, because this tank is empty most of the year and the money is better spent on the test kit.

Fifty watts is the correct wattage for a 10 gallon tank by our usual arithmetic, and a quarantine tank is almost always 10 gallons or less.

A detail worth getting right: match the quarantine tank's temperature to your display tank rather than to a number on a chart. The fish is going to be moved between the two, and a temperature step is a stressor you can simply not inflict.

The honest costs. It is preset, so you take the temperature the maker chose and cannot dial it to match your display tank exactly — the single biggest argument for spending more on an adjustable unit if your main tank runs warm or cold. No thermometer is included, so the setting is a claim rather than a reading. And a glass heater in a bare tank with nothing to brace it against is easy to knock.

A preset heater states a temperature. A thermometer measures one. Those are different claims, and the gap between them is exactly what a stressed fish experiences when it is netted from one tank into another.

Buy two if you are being thorough, or move one between the tanks and let each reading settle before you transfer anything.

The honest costs. A digital probe reads one spot, and a small bare tank with a single sponge filter genuinely does have gentle circulation, so give it a minute. The suction cup fails eventually and the unit ends up on the glass bottom. Button batteries are not replaceable on some models, which makes it a disposable device. And a cheap thermometer is better at telling you the water is stable than at telling you precisely what it is stable at — which is fine, because matching two tanks is a comparison, not an absolute.

This is the one entry on this page that is not optional, and the reason is the failure mode described further down: a quarantine tank with ammonia in it is worse for the fish than no quarantine tank at all.

A bare tank has very little biological capacity beyond what you moved across on the sponge. It is a small volume holding a fish that is probably not eating normally, and it has no plants and no substrate to buffer anything. The only way to know it is safe is to test it, daily, for the whole quarantine period.

The honest costs. It does not test hardness, chlorine or alkalinity. It is slower and fiddlier than strips, the nitrate test needs the second bottle shaken hard and then a wait, and the color card is genuinely hard to read between adjacent shades under warm light. Reagents expire, so a kit that has been in a cupboard for two years deserves suspicion before an important reading.

A quarantine tank is run by water changes more than by filtration, and that makes a conditioner a consumable here rather than an occasional purchase.

The ammonia-binding claim earns its place in this specific tank. If a reading does creep up between changes, the 48-hour window Seachem describes is time to act in rather than a fix — and in a small bare tank, readings creep.

The honest costs. It does not remove ammonia, it binds it, and your test kit will still read it afterwards, which is confusing the first time. The window is a window, and dosing it instead of changing water is how fish end up living in slowly worsening conditions. It smells strongly of sulfur. And none of this substitutes for the water change itself.

Included because the honest cheapest route — a bare 10 gallon tank from a local shop — is not something we can link you to, and a reader who wants to solve this today needs an option that ships.

Five gallons is small for a quarantine tank and fine for the fish most beginners buy: a betta, a few nano fish, a single new addition to a community tank. The lid is the part that earns its place, because a new fish in an unfamiliar bare tank is exactly the fish most likely to jump.

The honest costs. No heater, no test kit, no substrate and no dechlorinator, so this is a tank rather than a setup. Acrylic rather than glass on this class of kit, which scratches far more readily when you clean it, and scratches are permanent. Koller publishes no filter flow figure, so the filtration cannot be checked against our turnover arithmetic before you buy — though in a quarantine tank you will likely run the sponge filter above instead. And five gallons leaves a small margin for error in every direction, which is why the daily testing matters.

The skip entry, and it is a good product in the wrong tank. Gravel is the right floor for a display aquarium and the wrong floor for a quarantine tank, for three reasons that all point the same way.

A quarantine tank is meant to be readable. Bare glass shows you uneaten food, waste and anything a fish passes, immediately and without a torch. Gravel hides all of it between the grains, which is the one thing you did not want here.

It is also meant to be emptied and dried between occupants. A bare tank is rinsed, dried and put away in ten minutes. Substrate is the part you cannot realistically sterilize, so it becomes the thing that carries something from one quarantine period into the next.

And a bare bottom makes the daily water change trivial — siphon straight off the glass, no vacuuming technique required.

The honest costs of the skip. A bare tank is a barer environment, and some fish are visibly more settled with cover, so give the occupant something to hide in — a ceramic cave, a length of pipe or a clump of a plant you do not mind discarding. That is cover, not substrate, and it comes out with the water.