Neon Tetras Fighting

Yesterday I noticed my 2 neon tetras having a bit of scrap. Today I noticed one of them with a damaged fin. Is this normal behaviour ?

Or could this be down to the fact that there is only 2 of them ? I did start with 10, about 2 years ago. but sadly lost 8 of them over time.

Also i have noticed one of my dwarf corys could be hurt as well, or am I overthinking ?

Managed to get a closer picture of the Cory. he does look like he’s taken a beating on the side

More than one recent scientific study has determined that shoaling/schooling fish need a group of close to 10 or more. One study tested the species in individual groups of three, five and ten. The groups of three and five had increased aggression, the groups of ten did not. The smaller groups also displayed a latency to feed. And for fish to be reluctant to feed, it shows they are significantly affected.

I believe you had an earlier thread on pH in which you mentioned the 2 neons left from the original 10. Replacing the group is obviously one option, or leaving the two and going with another species. Sometimes when one, two, or three are left, as when the others naturally die off from old age after a few years, the remaining fish may or may not show signs of stress from this. I have had it go both ways. I had one Red Phantom Tetra left from the original 15 and this last fish was in the tank for 3 years before it too died. On the other hand, my surviving five pencilfish from a group of close to 20 for many years became real terrors, tearing into each other and the rummynose, so I euthanized them.

Given the aggression obvious here, one neon will soon be on its own. But it may also attack any other fish, for the same reason. If this were me, with this small tank, I would euthanize the neons and move on. I know it is not easy, but you absolutely cannot have any aggressive fish bullying others, it is severely stressful and they will then themselves succumb to this or that. And, when any fish has developed symptoms of aggression, it is never reversible because the effect has harmed the fish itself.

As for the cory, there is something but I never guess with disease issues, so hopefully other members will be able to pin this down. It could be an attack from the neon, or disease.

@DoubleDutch @GaryE

Yeah I did think that the problem would be due to the fact they are not in a school. However they have not once turned down food. and both roughly get the same amount of food each time.

Yeah i definitely dont want to add more neon tetras. and i’m going to hang on from adding anything else to my tank until I know the pH and GH are are back to what they should be!

I definitely don’t want to leave them in my tank if they are going to hurt my corys, but I don’t like the idea of euthanising them. How would you even go about doing this ? I would prefer to take them to my LPS. Or would that be a bad idea ?

That's a male neon, so if the other is too, that could explain it.

I think the Cory has a different issue - it looks like a lump to me, and that could be a lot of things.

sam_mitchell98 said:

Yeah I did think that the problem would be due to the fact they are not in a school. However they have not once turned down food. and both roughly get the same amount of food each time.

Yeah i definitely dont want to add more neon tetras. and i’m going to hang on from adding anything else to my tank until I know the pH and GH are are back to what they should be!

I definitely don’t want to leave them in my tank if they are going to hurt my corys, but I don’t like the idea of euthanising them. How would you even go about doing this ? I would prefer to take them to my LPS. Or would that be a bad idea ?

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My mention of feeding was only to show how fish can be affected. The aggression is certainly the prime result from the studies into this, and the latency to feed was observed as a secondary result.

I cannot tell you what to do, I can only point out the science behind all this. You cannot have an aggressive fish attacking other fish, this is not going to end well. It weakens the other fish's immune system and they become susceptible to other problems, which may well be the cory issue.

The few times I had to euthanize fish, I followed what has been shown to be the quickest and most humane method. Net the fish out onto a folded over paper towel, quickly fold it several times, and give it a hard thump. This kills the fish instantly, which cannot be said of any other common method like clove oil, freezing, or whatever.

GaryE said:

That's a male neon, so if the other is too, that could explain it.

I think the Cory has a different issue - it looks like a lump to me, and that could be a lot of things.

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Yeah the other neon is of the same build, so I would sat they are both male. Can you see the second image I posted of the cory ? its more of a ‘dint’ rather than a ‘lump’ ?
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Byron said:

My mention of feeding was only to show how fish can be affected. The aggression is certainly the prime result from the studies into this, and the latency to feed was observed as a secondary result.

I cannot tell you what to do, I can only point out the science behind all this. You cannot have an aggressive fish attacking other fish, this is not going to end well. It weakens the other fish's immune system and they become susceptible to other problems, which may well be the cory issue.

The few times I had to euthanize fish, I followed what has been shown to be the quickest and most humane method. Net the fish out onto a folded over paper towel, quickly fold it several times, and give it a hard thump. This kills the fish instantly, which cannot be said of any other common method like clove oil, freezing, or whatever.

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oh yeah I understand. I was just saying feeding hasn’t been an issue. obviously the aggression is still there.

I’m greatfull for you sharing your knowledge, and will always take on everything that is shared from yourself and the other experienced fish keepers on this forum. I will 100% be removing the neons from this tank. one way or another.

Maybe euthanising is my only option right now, so thanks for the tip. I definitely won’t be trying anything like the clove oil or freezing.

Byron is an expert, so his opinion carries more weight, but I would personally return them to the LFS. Since the neons behavior is environmental (not enough fish) rather than physiological, it should improve when they are in a large school.

Byron said:

And, when any fish has developed symptoms of aggression, it is never reversible because the effect has harmed the fish itself.

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I've never heard of this. Is this from a scientific article or personal experience? I've found that changing stocking or tank lay-out can decrease a fish's aggression.

otterblue said:

Byron is an expert, so his opinion carries more weight, but I would personally return them to the LFS. Since the neons behavior is environmental (not enough fish) rather than physiological, it should improve when they are in a large school.




I've never heard of this. Is this from a scientific article or personal experience? I've found that changing stocking or tank lay-out can decrease a fish's aggression.

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There are different reasons for aggression. What we are concerned with here is a physiological change in the fish. This is due to the severe stress of not having a group. The expectation is programmed into the species' DNA. We cannot change DNA. The fish expects a group, and scientifically controlled studies have clearly shown that the smaller the group the more it affects the fish. As I said earlier the very fact that the fish can become hesitant to even eat shows that it is a very serious issue to the fish.

This is not the same as becoming aggressive as a fish, say a betta, would to other fish in his territory. Once the other fish are gone, the betta is not still being impacted. And this aggression is natural, induced by some external issue such as another fish in "his" space. The betta will defend its territory, but once the threat is gone, the betta is no longer being threatened or stressed out by it.

Moving the aquascape around sometimes works, sometimes not, but not when it comes to a detriment that causes the fish extreme anxiety. Maintaining a shoaling fish without sufficient numbers of its own places the fish in what it views as an extremely dangerous environment. The fish is under fear because the natural defense of the group is not present.

We know that many animals will "crack" from this sort of negatively. They often use aggression because they have no other means. It is cruel and inhumane to keep such a fish contrary to what it expects.