Aggressive Zebra Danio

I'm pretty sure my one Zebra Danio is attacking the other Danios in my tank. I thought they were calm and fun schooling fish. The one Danio aggressively swims all over chasing the Danios around the tank until he corners them and looks like nips at them. We recently had a Danio die who had a strange pale spot on him that my brother swore was a bite mark. I laughed him off saying Danios don't do that. Until today I'm watching the aggressive Danio chase another Danio around the tank trying to catch him. And now there's another Danio with the exact same marking on him that I'm starting to agree looks like a bite. I'll try to snap a photo if I can.



It's not a raised mark or fungus or cottony looking. It's indented

 

What size tank? How many zebra danios?

 

What size tank? How many zebra danios?

20 gal. We had more Danios but with old ammonia issues we lost some and haven't replaced the school. We wanted to get the water safe and have been putting it off. We only have 3 now.. had 4 until the other one we are pretty sure was attacked and died

 

Zebra danios are schooling fish and must be kept 6+ of it's own species in a tank. 4 zebra danios isn't enough for a danio school, so that was probably why it attacked.

What temperature do you keep the tank at? Any other fish besides the danios?

Since when did the attacks start? Once you got them?

 

Since when did the attacks start? Once you got them?

We got the aggressive Danio maybe 2-3 weeks ago? I think he was rather spunkier than the rest but the real feisty stuff probably started 1 or 2 weeks ago

 

You said you had six and now you have 3. So, were the two other deaths caused by the aggressive danios?

 

You said you had six and now you have 3. So, were the two other deaths caused by the aggressive danios?

Pretty sure one was an ammonia related death. And another "disappeared". We searched the whole tank, moved items, checked filter etc. I'm assuming it died somehow and was eaten up after before we saw it. I read about Danios jumping out of ammonia spikes but we have a lid so I don't see him making the jump perfectly out the food hole of the lid

 

don't see him making the jump perfectly out the food hole of the lid

You would be very surprised. Ive had fish jump out of some really small holes.

I also agree with everyone else to try and up the school.

 

I would definitely up the school. In my experience with zebra danios, they are aggressive when not in groups of 6+, and peaceful when in groups. A similar thing happened to one of my zebra danio schools in the past as what has happened to you.

 

Yeah, so after the two deaths, you came down to 4. Since 4 isn't enough for a school, one of your danios started to be aggressive. I will up the school to 6+