Glofish Tetra chasing, bullying or attacking

I have 4 glofish tetras and I just moved them yesterday from a 10 to 20g tank. They’re all different color - red green yellow and blue. They all seem to be chasing/nipping at the yellow one. Especially the blue. It seemed to be playful at first but I’m now noticing the yellow breathing super heavy and at one point was motionless for a minute toward the bottom of a Java fern. Seemingly either resting or hiding. But the blue one finds it and keeps chasing.
I even took each one out for a few minutes just to break them up, but they’re back in and still going at the yellow.
Are they marking territory? Could the constant chasing kill the yellow? Any suggestions?

 

Tetras are not peaceful in their species. Black skit tetras (the base for your GMOs) are no exception. It's completely normal for tetras to establish small display territories.

Are they marking territory?

That and making out the pecking order.

Could the constant chasing kill the yellow?

Not by itself, but the stress may make the fish vulnerable to disease and parasites.

Any suggestions?

Structure the tank. Rootwood, twigs, branches, plants. And cover from above from floating plants and the like.

But although you can reduce the behaviour it will never completely stop, so get used to a minimum level of bickering.

 

Thanks very much! Will adding more of them, or like two of the same color help also? I think we have room for a couple more, it’s a 20g long.

 

Depends on what and how many fish you keep with them. The colour is irrelevant, they are all the same species. But some more might be a good idea. If they are the only fish 2-3 more would be ok. Make sure they are roughly the same size.

 

The inly advice I have is lots of plants to break visual line and perhaps a larger group to spread the wealth. They are best in a school of 5+. When I had tetras we had some chasing like this. The color is irrelevant. Black Skirt Tetra Care - Size, Life Span, Tank Mates, Breeding