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How does fish immunity actually work?


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Hello everyone!


There is a common topic that is named as "bacteria conflict" in my country which is believed to be more common beyond guppies, discus and angelfish. They suggest not to mix these fish from different sources and lean towards locally bred species. Some claim their "bacteria" conflicts with each other resulting in deaths. Some think this is a misconception of discus plague. I think this is all about immunity.



So, whenever we get a new fish, we *ideally* quarantine them, acclimate them to their new environment, check for their care guides, learn from experiences, prepare a tank to meet their needs... and so on


But one thing I think we don't pay enough attention to is their immunity, pathogens and how they will handle going into a community tank after completing all the quarantine and stuff successfully.


How does fish immunity work actually? For instance, when we get a wild-caught fish or fish from farms (which mightve raised their fish in brackish conditions, thus their immunity might not be used to even common stuff to handle, or is already used to water and conditions of a faraway land), how can we make these fish get used to their new environment actually? 

Besides a good diet, environment, low stress, and keeping the tank between ideal temperatures, can we do anything else do build some immunity? Like adding some floating plants or adding some water from the main tank to the quarantine tank slowly like once a week to build up resistance by going slow? What would be the safest way to build up the immunity system before completing a quarantine and directly being added to a new tank?

(I know these fish go through stressful times - at least transportation all alone- and likely end up in many different tanks before ending up in our tanks, but I would love to learn more about immunity and what we can do to build up some resistance. Like if I wanna keep my wc fish in a community setup, what should I do?


Tagging the three 😝🥰

@Odd Duck @Colu @Biotope Biologist 



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On 12/18/2023 at 12:36 PM, Lennie said:

Like adding some floating plants or adding some water from the main tank to the quarantine tank slowly like once a week to build up resistance

I do this.  
I strongly believe that especially among things like guppies imported. They are raised in brackish water and like you said have built no immunity to common tank bacteria. 
Often starting with a touch of salt in a qt with fish then slowly changing it out then slowly adding water from the min tank they will live in is a way I like to do it.  
That way if they are going to be negatively impacted they can adapt slowly or be treated until their immune system can handle whatever exists in the main tank that my fish dont even notice. 

Great topic 👍

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So many smart people around here!  Excellent questions and topic.

There is always risk of introducing new potential pathogens (whether they are bacterial, viral, fungal, or parasitic) when moving fish between tanks.  Even more so moving them across the country or across the world.  The most important thing is to do everything you can to maximize overall health since that will support their immune system.  There’s some evidence that having beneficial bacteria around can stimulate the immune system to work better.  Doing a gradual exposure is smart and may be helpful to act a bit like a vaccine / inoculation so the fish’s immune system can learn how to deal with the microlife in your tank(s).

Wild caught fish would likely be well-served by getting a full round of deworming and certainly a solid quarantine time is the best way to spot if there are issues and allow for treatment time.  I’m also a believer in making the quarantine tank as “homey” as possible giving the fish hiding places and plants as appropriate to minimize stress.  If I have to discard or disinfect everything afterwards, I will.

So do everything you can to keep the fish healthy and you are supporting their immune system.

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If I engage from my field, I would say we are kind of still unsure. There are many many fish health and medicine studies all around the world being conducted as farm fish provides humans the easiest form of protein with the best ratio of feed to edible protein. 

But farm fish all have a common problem that we see in our fish tanks. Once you take a fish from the wild and put them in an enclosure they become more susceptible to bacterial, fungal, and parasitic interactions. Even if you ship them with water from their native land this happens. I wouldn’t even hazard a guess as to why this happens from a biological point of view.


But I notice that when I keep my fish in a tank where I allow the flora and fauna to flourish and the mulm to buildup versus a sterile environment with no substrate and constant turnover of water I have a easier time keeping them alive. They get fin rot, some sort of bacterial infection on the gills or in the marine world eye infection if kept in these sterile conditions . Perhaps it is similar to the theory that the children who eat dirt and bugs snd play outside have a stronger immune system because their immune system is engaged with threats constantly and thus is better at identifying actual threats versus perceived threats? 

Suffice to say I don’t know. I think if we did know the aquarium industry would be better for it, not to mention aquaculture.

Edited by Biotope Biologist
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@Lennie, to your point, I once had a quarantine tank holding about 20 juvenile South American cichlids. I remember they were in QT for about 3 weeks without issue when I added one more fish who turned out to be extremely aggressive. He was there but a few days and was terrorizing the tank and the entire tank came down with the worst case of ich I had ever seen. All but the aggressor caught it.

Also, to your point, my severums came down with what we ultimately believe was probably viral lymphocystis. A full 6 months of nearly every available antibiotic, dewormer, and anti-parasitical drug had no effect, nor did salt.

The illness certainly had all the visual hallmarks of lympho as well as the symptomology. Although it was unsightly and oft times made it difficult for them to swim, it never killed any severums.

At this time, @Guppysnail, @Odd Duck, @OnlyGenusCaps and I were experimenting with aquarium cycling acceleration techniques and beneficial bacteria products, and it finally dawned on me that I could try a probiotic approach on the alleged lympho. Why not, nothing else worked.

I dosed the severum tank daily with 10X the normal dose of BB. No fish seemed to notice. It had no ill effects and in fact, it made the water extraordinarily clear.

Prior to this, the full 6 months of treatments included:











Malachite Green/Formalin

Methylene Blue


And I’m probably forgetting others we tried, also to no avail.

The University of Florida published a study on anti-viral treatments in fish and provided these images of a lympho-infected fish (left) that appeared similar to what I see on my severums (right):

Picture1.jpg.daa7370988c3bbce7eaecb637745af0c.jpg    13.jpeg.e14d242e7bf88284af7c8089c4e542d7.jpeg

Their microscope image of the infected and distorted cells is quite similar to mine although UNFL’s image (left) is much higher quality than mine (right):

lymphomicro(3).jpg.3d8ab2db3ff4ff76197f8b0dcd7d4704.jpg  LYMPHOCLUSTER.jpg.676b0d4d3ebb3a208b4aa14d3ca631a4.jpg

But inside of a week of inundating the tank with beneficial bacteria, this happened:

DAY-12.JPG.8d84690f6acea224fd14ef5f1dd829f1.JPG  Day-3.JPG.91d985983d45e7d214335d82d7695ac3.JPG  Day-5.JPG.6fc6977713caa49f280aeaca3b7f43ab.JPG

                                              Days 1-3                                                             Day-4                                                                  Day-5

Where 6 months of drugs failed; a week of BB arrested what we think might have been lymphocystis. Why did it work? I wouldn’t know. I know next to nothing about biology. To that end, and this is pure speculation on my part, but my thinking was to ‘out compete’ the pathogen with a harmless bug. I guess dumb luck is still luck! 🤣

I consulted my friend @Odd Duck, and she suggested what we might be looking at here was the denial of binding areas; basically, the population of ‘real estate’ by the BB that therefore denied areas for the pathogens to attach and reproduce.

I was also discussing this with my friend @Guppysnail to perhaps create an article about it but I decided not to as I’m ill-equipped to do more than speculate. In addition, I had no way to verify that this indeed was lympho.

But what about immunity? What we all noticed when applying BB therapy was often the fish seem to have increased color and energy. Greater appetites and increased frequency of breeding. Water clarity improves as well. We have no way to assert with any authority that it did indeed improve their immunity nor anything definitive about the aforementioned binding area concept. 

But whatever it was, the affliction resisted months of antibiotics and anti-parasitical drugs, yet we saw it arrested in just days by repeatedly dosing the tank with BB.

So, while not particularly useful, given the lack of verifiable data, I thought it was interesting and to your point @Lennie.


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