Black Neon Tetras killing each other?

[Mikon77]

Hello! About a week ago I added my first fish to my aquarium (20 gallon high) after the cycle was complete. I added 6 black neon tetras. The Petco employee told me to get 4 more after a week has passed.

Fast forward to yesterday and I get 4 more and introduce them into the tank. I immediately notice some bullying. There was some head-butting and ramming, followed by some of the tetras staying away from the group. However, this morning I found one dead. I took it to Petco with a sample of my water and they said my water was perfect and to keep the school at 9 instead of 10. Unfortunately, as soon as I got back from Petco I found a second one dead. It was alive when I left, and I was only gone 30 minutes.

Are they killing each other or is something else wrong? There appeared to be no signs of infection on either of the dead fish and they are all eating well.

My water parameters are 

Ph: 7.8
Ammonia: 0
Nitrite: 0
Nitrate: 10-15ish
Gh: 10
Kh: 3
Temp:76.5

Edited April 14 by Mikon77

[Whitecloud09]

@Mikon77, that can be a sign of overcrowding or even they are all males and they are super aggressive. What other fish are in the tank? Or are they the only ones?

 

[Mikon77]

On 4/14/2024 at 3:51 PM, Whitecloud09 said:

@Mikon77, that can be a sign of overcrowding or even they are all males and they are super aggressive. What other fish are in the tank? Or are they the only ones?

 

I’m not sure what gender they are, my local Petco keeps them all together. They are the only fish in the tank. I’m hoping it’s not because I went with a 20 gallon high instead of a 20 gallon long. By the time I heard a 20 gallon long is better, this tank had completed its cycle. The Petco employee told me that 10 tetras would do just fine in a 20 gallon high, and no more. Here is my tank now.

 

I’m worried now because I’d like to add some Amano shrimp and one dwarf gourami, but not if my tetras will kill them!

Edited April 14 by Mikon77

[AllFishNoBrakes]

My wild caught black neons are super chill. They all hide together, and then school out together, and then dart back behind the plants together. 
 

In my experience they’re super docile. I have 22 of them in a 29 gallon blackwater tank. 

[Mikon77]

On 4/14/2024 at 5:42 PM, AllFishNoBrakes said:

My wild caught black neons are super chill. They all hide together, and then school out together, and then dart back behind the plants together. 
 

In my experience they’re super docile. I have 22 of them in a 29 gallon blackwater tank. 

Mine have been nothing but aggressive! They will school for a little while, then go back to attacking each other. This morning I woke up to a third one killed. This one had slightly visible wounds. I’m now down to 7. I really hope it’s not because of my tank size!

[Whitecloud09]

Wierd, i would add more decor and hiding places for them and tall plants so they can dodge each other @Mikon77. So odd......