To answer that question, here’s a horror
story courtesy of Koran Weston, a fishkeeper
from British Columbia, Canada, who has
three freshwater community tanks: a 75-
gallon, a 25-gallon, and a 10-gallon.
One day she got a hankering for a
paradise fish. So she picked one that looked
plump and healthy and added it to her 10-
Everything went fine for about a month—
until the day that Weston noticed a red
worm protruding from the vent of that
plump, healthy-looking paradise fish. It was
a camallanus worm, a nasty and contagious
nematode that lives in the intestines of fish.
It hadn’t been evident when she bought
the fish, probably because there are several
stages to the parasite’s lifecycle, and only the
last one is visible to the naked eye. But soon
it was wreaking havoc in all of her tanks.
By the time the outbreak was under control
three months later, she said, she’d spent
some $200 on medication and lost dozens of
fish, including her three beloved angelfish.
“Now I quarantine all fish for six weeks.
No exceptions,” said Weston, who has a 5-
gallon tank specifically for that purpose.
That’s music to the ears of Dr. Tim
Miller-Morgan, an aquatic veterinarian who
runs the ornamental fish health program
of the Oregon Extension Sea Grant. “I’m
constantly ranting about quarantine,”
Miller-Morgan said. “It’s very underutilized,
and yet it’s just essential.”
He explained that even healthy fish carry
some pathogens; the reason they don’t
get sick is because their immune systems
are able to keep them in check. But when
fish are placed under stress—as they are
when they’re bounced from fish farm to
wholesaler to retailer to a hobbyist’s tank—
their resistance often drops, and pathogens
can gain the upper hand.
Is Long Enough?
How long it takes for this to happen,
though, can depend on a number of factors,
including the species of fish, the pathogen,
and the water conditions both are living in.
For instance, at temperatures of 75° to 79°F,
the life cycle of the parasite that causes ich
is about 48 hours, while at 60°F, it can take
nearly a week to complete.
And that’s where quarantine starts to
get complicated. If every pathogen has its
own life cycle, and the life cycle is often
dependent on water conditions such as
temperature, then how do you know how
Kwest Art (Shutterstock)
That innocent-looking carnival goldfish can be a ticking time bomb if added straight to an
long of a quarantine is long enough? Is two
weeks adequate, as some fishkeepers insist?
Or three weeks? A month? Six weeks?
“There is little scientific evidence on what
is the ‘ideal’ period,” admits Dr. Edward Noga,
author of the textbook Fish Disease: Diagnosis
and Treatment (Blackwell Publishing, 2000)
and a professor of aquatic medicine at North
Carolina State University’s veterinary college.
“In general, a three- to four-week quarantine
period is considered good, but [it] depends
upon the particular pathogens that pose a
risk to the population.”
Miller-Morgan said he knows some koi
hobbyists who quarantine a new fish for
a full year before adding it to a pond
with established stock—“They don’t feel
comfortable until they put it through
a year of temperature fluctuations,” he
explained—but in general he thinks a
month is a reasonable amount of time to
isolate new arrivals.
“Quarantine isn’t just to prevent
disease from getting into the system,”
he said. “It also lets the fish acclimate
to new surroundings, a new system, new
food. It gives them a chance to settle
down and gives their immune system a
chance to recuperate [from the stress of
And by the way, while most of us tend to
think the point of quarantine is to protect
our established stock, the reverse can also
be true. The fish in your tank are used to
certain pathogens that are already there,
and have resistance to them, but a new
fish added to that tank might not, Miller-Morgan explained. By isolating it, then
adding small quantities of water from the
established tank to the quarantine tank
over a period of several weeks, you’ll give
the new fish the opportunity to build up
resistance to any pathogens that might be
present in that water.