Mr. Saltwater Tank

Terrible Advice Tuesdays (T.A.Tues.): Semi-Solitary Confinement Works Great For Fish Quarantine

Terrible Advice Tuesdays (T.A.Tues.): A separate tank plumbed into your display tank works fine for a quarantine tank.

The rest of the story: Placing a fish in a separate tank, yet having that tank plumbed into your display tank such that the tanks share the same water is not true quarantine. The reason is that since the tanks share the same water source, any disease that could be on the fish in “quarantine” can pass into the display tank. Even if you use foam pads, filter socks or ultraviolet sterilization between the tanks, the disease can still cross contaminate the display tank.

To quarantine a fish properly, the quarantine tank and the fish have to be kept separate from the display tank. Any water, filters, nets, etc that come in contact with the quarantine tank cannot go back to the display tank otherwise cross-containation can occur. I’ll add that there is certainly no problem using display tank water to fill or to perform water changes on your quarantine tanks. Just make sure any equipment that touches the quarantine tank is kept separate from your display tank. If the equipment is going to come in contact with your display tank, it must be sterilized first!

Finally, keeping a fish in a separate tank that is plumbed into your display tank is a useful way to train a fish to eat certain foods. When the fish is in the separate tank, it won’t have to compete for food with the other fish in your display tank. The fish will still need to go through the full quarantine process before it gets placed in the tank though. Of course, while the fish is in quarantine, that’s a great time to train it to eat the food so why not double dip!

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Comments for this article (21)

  • Grant says:

    Thanks for the advice. Right on time to. I am starting a quarantine tank this week cause I would like to get a purple tang for my second tank (can’t have just one). Looking forward to the new member of the family.

  • Matt says:

    Hi Mark,
    If you say a seperste net should be used for quarantine tanks, then how do you net a fish from your Q tank to your main tank or vice versa? Isn’t cross contamination inevitable in this scenario?
    Also what is the best way to sterilize equipment?

    Best regards. Keep up the good work. Love all the advice.

  • Matt…Once a fish has passed through quarantine successfully, then you can assume it is safe to put in your tank. Therefore the net you touched the fish with is ok to touch your display tank. Remember that once a fish starts quarantine, then any new fish that you add to that quarantine tank reset the 30 day clock.

  • Jerry says:


    You should still acclimate any fish that go from your qt to your dt. I only mention that due to the net question. The water parameters can be and usually are vastly different, especially with a reef tank. I tell people all the time: Don’t potentially kill a fish that you’ve spent good money and a month of time on because you want it in your display tank 30-60 minutes sooner

    Not saying you don’t, just thinking that the net wouldn’t be an issue if you were

  • kevin says:

    Imho such tanks are great for strenghtening fish. Either because they were bullied by other fish (or the bullies). And also for frags (if you light them).

    Same water is less stress for any animal. Just make sure the fish or whatever is healthy before you put them in.

  • JasPR says:

    Sounds like basic truth and the stuff of common sense. But many a newbie comes to this hobby with no background in science or animal husbandry so good 101 information.
    In addition to the basic physical separation of ‘unknown’ individuals, and in line with Mark’s fundamentals, never put your quarantine next to your sump or display aquarium if you use air stone, ehiem spray bars and anything that creates a mist or spray when water level is lowered. That spray can transfer disease just as a net or plastic cup can. For what its worth, JasPR

  • Mark says:

    Love your advice but I have to disagree on this one. I have my qt tank plumbed into my system. It’s a 40 breeder with over 50 watts of uv. Once a new fish is added to the qt tank, I isolate the fish for 30 days but shutting off the supply line from the system. Then do 5 gallon daily water changes with system water. Once the fish looks clean for 30 days, I will slowly bleed him in for 2 weeks. I have a total of 500+ gallons of water on my system. Some fish include
    Powder blue tang
    Achilles White Cheek Hybrid tang
    Clown Tang
    9 leopard wrasses and many other sensitive fish.
    No signs of ich or other nasties.

  • are essentially doing what I recommended in the post. Your fish is isolated for 30 days since you stopped water flowing through the quarantine tank. There is no exchange between the tanks until the fish is done QT’ing

  • Jose says:

    Awesome Mark. As you know im bulding a tank to the dimmensions similar as yours. So in order to start filling up tank with fishes, and as not to do it one by one, is there like a suggested amount you should quarantine at once and then put them all in to the Main tank? thanks. I mean buy 3 and after 30 days but the 3 in the main display ,,,or 5 or 6 🙂

  • JasPR says:

    I think it would wise to do some research on ectoparasites and pathogenic bacteria of the digestive tract as a general primer.
    In addition to external infestations, every species of fish can be infected by unique forms of virus and especially herpes virus. These things are beyond the word ‘tiny’. Microscopic forms ( anaerobic bacteria counts) abound in closed systems in concentrations not typical in the vast ocean. And every fish that comes from the ocean and passes thru a holding or trans shipping facility comes in contact with EVERY fish that is in the system or has passed thru that particular system recently. Add the levels of stress and G.A.S. to the equation and you have IDEAL conditions for hitch hikers and carriers, as well as down right ‘mystery deaths’ in the journey to first a home system. ALL fish purchased WILL die in captivity. The drill should be to make that a rare, infrequent and non-contagious event!
    Marine disease/infestation diagnosis is difficult as the symptoms beyond the obvious ( cloudy eyes, congested fins, white spots and dusting) are subtly cooking and we simply see a fish that is thin, not eating and sulking– and then it is gone behind the coral and disappears.
    In hardier species like fresh water fish, we get a chance to see the diseases develop and come to understand the dynamic better.
    It is important to realize that many fish die from secondary diseases and domino symptom issues ( such as osmotic regulation issues) due to viral or bacterial issues in the internal organs. these fish shed the virus and bacteria constantly while they linger between life and death. And we often blame these things on collection technique, chemical poisoning, bad shipping etc. That can certainly be the case– or it can be something more hidden. Hopefully a ‘personal problem; and not something capable of spreading. Avoid the ‘hammer’ on the revolver— Quarantine! JasPR

  • watersnoopy says:

    How big should the QT tank? I am still puzzle by the dimension requirement.

    How big should the QT tank. I am thinking of getting queen butterfly fish. If that fish is in QT for 30 days, should it be at least in the 70 gallon tank?

    Please help me.


  • Chris says:

    I no longer have my reef tank due to living in rental houses yet still receive all marks videos and updates. Good to have someone around with so much knowledge and so much decency to share it with everyone. Champ!

  • Mo'Dayvia says:

    I’m so glad you said, this. I always told my friends that. The only good use of that tank being plumbed into the display tank is for feeding services, or aggression toward another fish, and birthing babies for freshwater fish. Great advice Mark.

  • Benjamin Jackson says:

    Mark (Mr. Saltwater), This sort of information definitely needs to make it to the general public, I can only think of the thousands of people who are making such intricate mistakes such as this and are heavily paying for it by losing money and or by losing their livestock which inevitably scares them away from the hobby.

    Mark (QT tank plumbed into my system),
    May I inquire of the details to how you have the QT plumbed into the main?
    This is something I have wondered about how it would be done so it performed properly and looked professional.
    If you like you may email me at with more details of your setup.

    Thank you both for your time in advance.

  • Mark says:

    Thanks for the response. It’s just that I have been hammered by my local reef club on my setup. They have a hard time believing that there are different setups regarding qt.

  • Jose says:


    Its just that it is possible, but it is a more advanced setup definitely. Most newbies would mix any kind of anything from one tank to QT by mistake, error, or even some water draining also by mistake.

    You only need to make the mistake once to put in danger Main Display tank

    So although it can be done, I personally think that If you can prepare your setup to avoid mistakes from the beginning, it will minimize chances of failure 🙂

  • Steve Dodd says:

    Mark , it always confuses me about the filter system on a quarantine tank, if you only buy a fish say every 2 or 3 months , would there be enough bacteria in what ever filter system you used to cope with the fish , it always says that the bacteria dies back when there is no food going through ( fish waste) it.
    so how do you keep the filter in full production mode with no fish in the tank ??????

  • Steve Dodd…it depends on how long you want the QT to stay up and running. When I’m not using mine for more than a week, I completely tear it down, wash it and store it. When I need to use it again, I’ll use display tank water to fill it and/or spike it with some Dr. Tim’s bacteria

  • Bryan Wells says:


    Do you recommend a bare QT with PVC only or do you recommend a tank with sand and live rock? I ask because I was going the route of a full-blown reef tank with live rock and sand and then I read a post saying keep it very minimal with no gravel and no live rock and just PVC for the artificial reef. If I do the bare-bones tank, does the Dr. Tim’s Bacteria still do any good? I got Dr. Tims before I changed my mind to go with a bare tank, and now I wonder if I should still use Dr. Tim’s, even though its not a true reef tank.


  • Bryan Wells…the QT tank needs to be bare bones…no sand or rock. PVC pieces provides hiding places. Hang on back skimmer works fine for filtration. You can still use the Dr. Tim’s to help with the cycling and you’ll need to watch nutrient levels closely as the tank doesn’t have a lot of biological filtration. Frequent water changes may be needed to keep the levels in check

  • JasPR says:

    absolutely, and if I might add? you can use ammonia ( lab grade crystals are available) to ‘feed’ the biofilter while the system is empty of live stock.
    I know its very hard not to buy fish on impulse as that is part of the fun of the hobby. But ideally, if we plan out our tanks and its future population in advance, it is a good idea to ‘batch buy’ your community residents. This way you have a good sized load in the Qtank to feed the filter with no nagging nitrIte appearing ( the whimpy bacteria of the two tag team microbes that break down ammonia to nitrAte). Feed the system for about two weeks with the ammonium crystals and test as if there were fish in it. Seed the system with YOUR sand or backwash from a filter. This is the VERY best seeding source as it is YOUR bacteria population adapted to YOUR raw water source. Works like a charm. For what its worth– JasPR

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