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I have three neon tetras and I'd like to add some more. My reading on the subject gives me different advice on what is likely to happen.

One outcome is the two sets of neons will not school together, the other outcome is that they will school together, but to varying degrees.

Are there any concerns with adding new neon tetras to an aquarium with an existing school?

EDIT: See my answer below for what happened when I added new neon tetras. See also this related question: how-long-should-i-wait-after-moving-aquarium-before-adding-new-fish?

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  • I've found that every time I add new tetras the original ones die within a few days. Why this happens is a complete mystery to me.
    – Mick Moore
    Commented Feb 18, 2020 at 14:10

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Neon tetras will school together irrespective to the order of introduction. The new fishes may hide and not eat for the first few days, but eventually even you wont be able to differentiate them. While not necessary, try to get same sized fishes as the existing ones. Acclimate them properly. Turn the lights off after releasing them into you tank.

My Neons even school with Cardinals. The same goes for Harlequin and Lambchop rasboras. As stated by Keltari in another answer, as long as the visual differences are not easily distinguishable, they will school together.

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    Why do you want the lights off?
    – Don Larynx
    Commented Dec 12, 2013 at 4:39
  • I believe this creates less stress on the new fishes, enables them to hide and feel safer.
    – fahad.hasan
    Commented Dec 12, 2013 at 5:16
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    Also if you acclimate your fish by floating the bags they're in, then they're floating at the top of the tank, right next to the light - which will heat up the bag.
    – Spidercat
    Commented Jan 10, 2014 at 3:14
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In my experience, neons will school at all times. It doesn't matter if they came from the same batch or not. When it comes to neons, the more, the better.

Even tetras of different breeds (as long as they physically look similar) will school together if there aren't enough of the same breed.

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Biggest concern is introducing a disease to your tank. Especially if you have a decent size group of them make absolutely sure the new ones aren't showing signs of ich or other diseases if you cannot quarantine them.

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To answer my own question then:

I added three new tetras, identical in colouring to the original three tetras. The additions were smaller than the existing ones.

Within two days, all three of the new additions were dead. I am not sure which of the other community fish (guppies, albino catfish, angel fish) were the main culprits. I did see the original neons occasionally bother the new arrivals, as did the guppies.

A complicating factor is that I had, earlier in the day, moved the aquarium from one room to another. I waited over six hours after moving the tank to allow the fish to settle down. All the original water was retained and returned to the tank after the move. I had assumed that leaving the tank for a substantial period would have allowed the fish to recover before adding new fish. Perhaps I was wrong.

I had also added two fancy-tail guppies to the tank at the same time as the three neons. These are doing fine.

EDIT: I added three more neon tetras, which I chose to be as close as possible to the size of the original three. One month later, these new additions continue to do fine.

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  • Neons are fairly dainty fish. If all 3 of them died, it was probably something environmental. It may not have even been anything wrong with the water, just something different about it (PH, for example)
    – GrandmasterB
    Commented Feb 19, 2014 at 21:20
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I did this when some of my neons died, I would recommend adding a few of the new ones as they can stay together until they familiarise themselves with the surroundings, in my personal experience after a month they were all schooled together no problem.

I would suggest getting the same species of tetra.

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I added 10 juvenile cardinals (generally thought to be more robust than neon tetras, and more colorful, with a complete iridescent blue line and more red). The existing four adult cardinals seemed to accept them without question or pause (this was after a period of acclimatization-of which I have learned that adding a small amount of existing aquarium water at a time to the acclimatized new school before releasing them assures a higher degree of acceptance (nitrate, ph level)). The problematic stage began after this, with the four adult cardinals dying one after the other, and this within a matter of two days. Generation assured? Passing of the flame? This is not meant to be humorous, but I can find no other explanation.

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I have 3 existing neon tetras and have a bag with 6 juveniles suspended in tank, slowly adding water. The existing tetras have been very curious about them and have been swimming alongside the bag with the juveniles mirroring their behaviour.

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    Has this method worked for you before? Is there anything a tetra owner should consider before combining new fish in an existing tank?
    – elbrant
    Commented Feb 15, 2019 at 1:34
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I see no problem with this at all but watch them carefully to make sure no aggression occurs. Also tetras are pretty sensitive, if you want to be really safe use drip acclimation.

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    Hi there, welcome to the pet stack exchange! Are you able to add more detail? Are there any PH changes that could happen, what about tank sizes? Is there any way to lessen chances of potential aggression. (Bigger tank for example.) Do you have a preferred method on introducing tetras?
    – SimplyRedAppaloosa
    Commented Feb 20, 2020 at 14:50

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