Aggressive Zebra Danio... Help!

I have had 6 zebra danios for a week now, the first few days were absolutely fine. A few days ago the thermometer popped off the side of the tank and was floating around in the water. The biggest danio was circling it and aggressively chased any other danio that came near it. I put the thermometer back and all was fine again. Then I added a fake floating jellyfish into the tank, the same fish did the same thing again so I took the jellyfish out. I have now noticed that the same danio won't let the others anywhere near one whole half of the tank. As soon as any of them get to the half way mark he chases them over to the other side, but strangely only when the tank light is on. As soon as I turn the light off it stops immediately. What can I do? Surely I can't just never put the light on?

 

I have the same thing in a community tank! Turns out my aggressive zebra danio is actually a leopard danio! I never noticed that till recently. He was purchased from a LFS with my zebras. I've spent lots of time wondering what makes him so "alpha" too. He likes to dart out of the school and go bully other fish larger that he is. He thinks he's bullying them but they're all fine. Probably not any help to you but point is the other fish probably aren't in any danger.

 

Welcome to FL

Zebra danios can be crazy fish. Putting them in bigger groups can help, as well as putting in decor and plants to break up the line of sight. Usually a bigger tank ( 20 + gallons or more ) is needed for fish that are so active like the danios and they should be in bigger schools ( at least 8 - 10 but preferably more )

I have several that just chase each other all day and fin nip the others. It's part of their spazzy nature.

What size tank do you have and what other fish are in it?

 

What size tank? Danios can be aggressive in smaller tank sizes.

 

My tank is 12 gallon. It' just the 6 danios in there at the moment, no other fish. The aggressive one is effectively isolating the other 5 into a space of about 6 gallon. He only does it when the tank light is on. He's completely calm and nice with the others in daylight and with the living room lights on. Could the bright tank light be stressing him out or anything like that?

 

My tank is 12 gallon. It' just the 6 danios in there at the moment, no other fish. The aggressive one is effectively isolating the other 5 into a space of about 6 gallon. He only does it when the tank light is on. He's completely calm and nice with the others in daylight and with the living room lights on. Could the bright tank light be stressing him out or anything like that?

Yeah, Danios are fast moving fish. Generally I wouldn't recommend them in anything less than a 20 long, even though they are small. If you keep the light on only for feeding, the fish will associate the light with food, thus making them aggressive. The light shouldn't be stressing him out. Overall I think you should return the danios and opt for tetras or fish that are more suitable for a smaller tank like that.

 

Yes 12 gallons is much too small for these guys, unfortunately. Then having them in such a small space and not in a good school, there will be fin nipping and aggressiveness happening. This is inevitable.

 

Give him his Jellyfish back.

 

They really need to be in a 20 gallon tank, and probably kept in odd numbers (fish, especially schooling fish, often do best in odd numbers, as it helps disburse aggression better - the exceptions are fish that pair off to breed).

Adding more décor to the tank will help, particularly plants or fake plants that go nearly to the top of the tank. But I would imagine that the best solution would be to get them into a 20 gallon tank.

Also for the record, "leopard danios" are the same species as zebras, just a different color variation.