How long should I quarantine new fish?
Until a parasite's life cycle has completed more than once without anything appearing — which is a date, not a number of weeks, because the cycle runs on a clock set by temperature. Put a fish into a quarantine tank heated to 78 °F and one cycle of freshwater ich takes about 5.5 days; three clear cycles is 17 days, with milestones on day 6, day 11, day 17. The same fish in an unheated spare room at 68 °F is on an 8.7-day cycle and the same three cycles take 27 days. Both are "about four weeks" in the folk version; only one of them is four weeks.
That is all this calculator does: start date, temperature, cycles → the date it clears. It is the arithmetic behind the quarantine tank guide, which covers the tank, the kit and the routine; this page only owns the calendar.
Why it's a date and not a number of weeks
The organism that sets the clock in freshwater is ich (white spot). It spends days embedded under the fish's skin where nothing in the water reaches it, drops off to divide on a hard surface, and releases free-swimming theronts that have 2–3 days to find a fish or die. A quarantine works because you are watching that loop run, and the only thing a "number of weeks" ever meant was a guess at how many loops it contained. Two published figures pin the loop's length: 21 days at 49 °F and 6 days at 76 °F (De-Hai Xu, USDA-ARS, in Tropical Fish Hobbyist). Fit a rate curve through them and the cycle speeds up 2.3× for every 10 °C warmer — an ordinary biological response, which is a decent sign the two figures agree with each other.
Everything on this page is that two-point fit: cycle length at your temperature, times the cycles you want, rounded up to whole days. It is a fit through two published figures, not a laboratory curve, so the ratios between temperatures are more trustworthy than any single day count — give yourself margin rather than counting to the last day. The figures cover 48–82 °F (9–28 °C); outside that the tool says it is extrapolating. Above about 86 °F (30 °C) the parasite cannot complete its cycle at all — that is the basis of heat treatment, a different subject from waiting a cycle out, and the tool stops rather than printing a number.
Can I quarantine for two weeks?
Only if the tank is warm enough, and the tool will tell you exactly how warm. Fitting three clear cycles into 14 days needs a 4.7-day cycle, which is 82 °F (27.5 °C) — near the top of what the fit covers and more than many species should be asked to take. Two cycles in a fortnight, the bare minimum that means anything, needs 73 °F (22.7 °C): an ordinary heated tropical tank. And in the unheated spare room at 68 °F, two weeks covers 1.6 cycles — less than one complete generation, which is a waiting room, not a quarantine. Two cycles in a single week would take 88 °F, past the model's range, and the tool says so instead of showing a date.
So the heater is a lever. Warm water holds less oxygen, so raise the air when you raise the heat — and do not push a coldwater species up there to save yourself a week; for goldfish and white clouds the calendar is longer and that is simply the cost.
What temperature should the quarantine run at?
The top of the species' comfortable range, not the top of the heater's. For most tropical community fish that is 78–82 °F: at 82 °F one cycle is 4.6 days and three clear cycles take 14 days, against 17 at 78 °F and 27 at 68 °F. That is a week or more bought with a heater setting, and it is the one place in quarantine where doing something actually speeds things up. The limits are the fish's, though. Warmer water carries less oxygen and raises a stressed fish's metabolism, so an air stone goes in with the heat; and a species that lives cool — goldfish, white clouds, hillstream loaches, most of the Corydoras that come from highland streams — does not get pushed to 82 °F to save you a week. Set the quarantine tank where the fish would be happy for a month, then read the date off the tool for that temperature. It is longer for cool fish, and that is simply the cost of keeping them.
Whatever the setting, hold it. Every number on this page is at one steady temperature; a small tank in a spare room that swings ten degrees between day and night runs its parasites on an average you have not measured. A cheap heater and a lid do more for the accuracy of the calendar than anything else in the kit.
The calendar at each temperature
| QT temperature | One cycle | 2 cycles | 3 cycles | 4 cycles | 2 weeks covers |
|---|---|---|---|---|---|
| 64 °F (18 °C) | 10.3 days | 21 days | 31 days | 42 days | 1.4 cycles |
| 68 °F (20 °C) | 8.7 days | 18 days | 27 days | 35 days | 1.6 cycles |
| 72 °F (22 °C) | 7.4 days | 15 days | 23 days | 30 days | 1.9 cycles |
| 75 °F (24 °C) | 6.3 days | 13 days | 19 days | 26 days | 2.2 cycles |
| 79 °F (26 °C) | 5.3 days | 11 days | 16 days | 22 days | 2.6 cycles |
| 82 °F (28 °C) | 4.5 days | 9 days | 14 days | 18 days | 3.1 cycles |
Computed by this page's engine; day counts rounded up. At 64 °F a fortnight is 1.4 cycles; at 82 °F it is 3.1.
The clock restarts from the last thing you saw
The start date is not the day the fish arrived; it is the last day you saw nothing. If spots show up on day nine, the day they are gone is the new day zero, and the calendar runs again from there — which is what the guides mean by "four to six weeks since the last symptom" without saying why. In the tool, change the start date and the milestones move. What this page will not do is tell you what you saw or what to put in the water for it: identification is a job for a species reference or an aquatic vet, and there is no medication dosing anywhere on this site by design.
They cannot be infected, but they can carry the cysts that hatch into theronts. With no fish to find, a theront dies within 3 days — so holding plants or inverts in fishless water for one full cycle plus that clears them: about 9 days at 78 °F by this page's engine, longer in cooler water. It is the same arithmetic as leaving an infected display fallow, which the guide covers.
This is freshwater ich — marine ich is a different animal
Reef keepers land on freshwater quarantine pages constantly, and the numbers do not transfer. Marine "ich" (Cryptocaryon irritans) is a different organism with a different and more variable life cycle, its own tolerance of heat, and a far longer fallow period than anything in the table above. This calculator's fit is for Ichthyophthirius only; rather than misapply it, the marine numbers are left out on purpose. If your tank is salt, use a reef reference for the fallow period and treat this page as an explanation of the method, not a source of the dates.
Common questions
Do the weeks the store held the fish count? Only if you know two things: the temperature of their system, and that nothing new went into that water after your fish did. A holding tank that received another shipment last Tuesday restarted every fish's clock on Tuesday. Since you almost never know either, count from the day the fish went into your tank — the store's weeks are a reason to feel better, not a number to subtract.
I ran a week at 70 °F and then turned the heater up to 80 °F. How do I count that? Cycles add, so two runs of the tool do. Seven days at 70 °F is 0.9 cycles; seven at 80 °F is 1.4; together that is 2.3 cycles, and what remains to reach three is a fraction of a cycle at the new temperature. Put the day you changed the heater in as the start date, take the cycles already banked off the target, and read the date.
Does the quarantine tank need to be cycled first? It needs to not poison the fish, which is a different bar. A sponge filter that has lived in the display for a few weeks brings a working colony with it; without one, a bare quarantine tank runs on water changes, which the water change calculator sizes and the guide's daily routine covers. Nothing about the calendar changes either way — the parasite does not care whether your ammonia is zero, only whether it has a host and how warm the water is.
What about treating them prophylactically instead of waiting? A different decision, and not one this site makes: there is no medication dosing here by design, and whether to medicate a fish that shows nothing is a judgment for a species reference or a vet. What the calendar still says is that a treatment course does not shorten the wait afterwards — nothing in the water reaches the stage under the skin, so the cycles you want to see go by cleanly are counted from the last day anything was seen, medicated or not.