One neon tetra behaving very aggressively towards others?

[nimreaper]

As the title indicates, I have one neon tetra amongst 10 that is constantly chasing and nipping at the other tetras.
There aren't any other species in the tank. Every time I look over at the tank, I can very quickly spot it chasing the other fish.
Oddly, he has a dark blue/purplish stripe instead of the typical blue-green stripe seen on the other tetras.

3 of my neons are OGs; I listened to poor advice from a fishstore employee and kept the 3 in a 3G tank. They fattened up and stayed there for a month before I could have the 20G long tank set up. I then moved the 3 to the 20G and purchased 7 more to make a total of 10 in the big tank.

I'm doing a fish-in cycle and have not seen this behavior until yesterday (before anyone comes for my neck like those on Reddit, I'm doing water tests every day at the same time; 6.6 PH, 0.25ppm or less Ammonia, 0ppm Nitrites, 10ppm Nitrates; dosing with Seachem Prime and Stability every day same time; 20% water change yesterday at the 1 week mark; planted tank with Amazon Sword, Bacopa, Roseafolia, Java Fern, Dwarf Hairgrass, Pothos, and Elephant Ear)

I've not had any spikes in water parameters. Temp is always at 76F. The fish were well-adjusted, active, and curious until yesterday when the harassment began. This bully fish is incessantly chasing all the other fish with very fast, jerky motions. I thought it might be male-dominance aggression but no other fish acts this way and he does it from morning until night, showing no discrimination amongst his brethren. This rogue fish does it regardless if there is food in the tank or not. I'm so worried he will cause the other fish to die from stress. One of the fat OG neon's even has a nipped fin from his antics.

I can't attach a video because my account is new. I made it so that I could interact on this forum.

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[nimreaper]

Should I just take him out and give him back to the fish-store? Sorry, but I really care more about the wellbeing of all my fish than one single bully fish, and it's not advised to keep a neon tetra by itself.

 

[Rbishop]

Are you purposely adjusting the pH to that low value? What kind of test kit?

 

[nimreaper]

Are you purposely adjusting the pH to that low value? What kind of test kit?

No I am not. The first two days it was 7.4. I haven't added anything new so I am not sure why it has slowly lowered (since day 3). I've read that neon's do best in slightly acidic water, so it hasn't bothered me too much.
I have an API freshwater master test kit.

 

[Rbishop]

What is your tap water pH on a sample set out overnight to gas off?

 

[fishorama]

Just to add to Bob's questions, I'll say tetras & many fish have a "pecking order". There is often a dominant fish that may pick on others. That's why it's good to have several of the same species to spread out any aggression. If you remove (rehome) the aggresser another fish may take it's place. 10 seems like a "safe" number but I'm confused by your "purple stripe" description. Could it be a different species? From your pics I see a male, female & another 2 males as far as I can tell...females are less likely to be aggressive...

 

[NoodleCats]

My experience with neons was a lot like yours... my entire group of 10 were hyper aggressive with one another and eventually turned their attentions to other fish in the tank. My black neons were terrified and constantly hid in the far upper corner away from them, my cories had fins shredded half the time.

The fighting among them was bad enough they bit the face off one of their own. It healed all gnarly.

I ended up rehoming them because even in a larger tank with a large group, lots of plants to hide in, nothing stopped their behaviour and it was really abnormal even for tetra squabbles.

Now I am very hesitant to try out any of the tetras from the same family group lol
My skirt tetras are less aggressive than those neons were, my emperor tetras too. And both of those are known to be more aggressive fish.

They dont even hold a candle to those neon tetras.


1. If you tank isn't heavily planted, try adding tons more plants and see if it helps your particular school. Like, very heavily plant. So planted you can't see the back glass of your tank.

2. Tank size is the minimum for neons, but maybe a larger tank can help. Mine was a 46 gallon for the record, still wasn't enough space for mine to be little tyrants peacefully. But, can always try.

3. Rehome. A gamble if you just rehome the one, as mentioned above it can just cause another to take its place. Or can rehome the whole group and try a different fish... I hear green neons are supposed to be a lot nicer. I dunno though, I won't touch green neons or cardinals thanks to my experience with neons.

 

[fishorama]

I had a similar experiences with silver tip tetras & cherry barbs, beautiful little fish but they didn't play well at all with each other in groups. I wondered about hormoning to make their colors pretty & maybe enhance commercial breeding. There's no telling what may be the problem but it seems worse in the last 8-10 years.

We've discussed this before NC I think. My experiences were less than yours but still alarming