Why Is My Neon Tetra Aggressive?

By: Billy | August 23, 2022 | Neon Tetras

For my first planted aquarium, I went with neon tetras.

I bought the 5-gallon Marineland Portrait tank since it was beginner-friendly.

After a few weeks, I noticed some of my tetras acting aggressively.

Why are they acting aggressively and chasing each other?! I’m here to outline some of the common reasons I found. Hopefully, this will help you solve your aggressive Neon Tetra issue.

Quick Answer

The most common issue is getting enough Neon Tetras so they feel comfortable schooling. They’re a schooling fish and can act more aggressively when their numbers are under 6. Your tank might also be too small for the amount you have in there.

Here are a few common reasons why Neon Tetra acts aggressively.

1. Not Enough

neon tetra

Neon Tetras feel most comfortable in a school. In an ideal world, you should have a minimum of 6 together. Most chain pet stores will sell them to you without providing this information.

For example, I naively went without researching Neon Tetras and got sold 5 for my 5-gallon tank. This was a big mistake. I immediately noticed they started to act aggressively toward each other.

2. Tank Size


Your tank size might not be big enough for them to school. I started with a 5-gallon Portrait, which was a bad idea. I should’ve stocked that size with a betta fish.

Neon Tetra like to swim in long, straight bursts. So a 5-gallon won’t be fair. They’ll do much better in a minimum size of 10 gallons. But honestly, getting a 20-gallon (or bigger) is better if you can.

Not only will a larger tank satisfy their swimming patterns, it’ll also allow you to add more. And as I mentioned above, they feel more comfortable in larger groups because they’re a schooling fish.

Even in my 20-gallon planted tank, they always don’t school. If you give them more space, they’ll feel more comfortable, and you’ll be able to see their natural behaviors.

3. Water Parameters

aquarium ph ammonia nitrite testing

Neon Tetras are from a tropical environment, specifically the Amazon river. You must do your best to replicate the conditions of their native environment. If you don’t, they can become stressed and start acting aggressively.

Ensure your tank is properly cycled. And your ammonia, nitrite, and nitrate levels are low.

Ideal conditions include…

If you want to get a better idea of their native environment, check out this YouTube video. I’ve found it best to watch/read about their native environment before building your tank.

4. Territorial

Sometimes it’s none of the issues above. You could have a perfect environment and still get some Tetras that act territorial. As long as they’re not hurting each other, it’s not the end of the world.

It can be extremely frustrating to do everything ‘right’ and still get them acting aggressively.

You’ve done your part. Sometimes it’s best to let nature run its course.

My Solution

Here’s what I did. Maybe not the solution for your situation, but still worth a share.

I bought a bigger/longer tank (UNS 60u – 20 gallon) and added a few more tetras.

There are now eight Neon Tetras in my 20-gallon aquarium.

They don’t fight anymore and while they don’t school like in their natural environment, you can tell they’re much more comfortable. The more Neon Tetra you can have together, the more natural their schooling behavior will be.


I've been building freshwater aquariums with my dad since a young age. A few years ago, I shifted my focus to low-tech planted tanks. My goal is to share my experience to help make yours easier.