Are Zebra Danios Meant To Do This?

[korok]

I completed a fishless cycle in my new 20gl tank last week so decided to start my stock with 6 zebra danios. I bought them on Tuesday, introduced them to the tank and they seemed fine, loosely schooling and being very active.

When I got up on Wednesday, only four of them were schooling and the other two were in hiding. After a couple of hours I was a bit concerned as the two were still hiding, but shortly after I turned the tank light on they came out and all six seemed happy enough again.

Fast forward to Thursday morning and again two of them were in hiding. So when I turned the light on a bit later I expected them to come out, but they haven't. And now the other four seem to be acting strange. Here is their behaviour for the last eight hours:

Danio 1 & 2: In hiding, didn't even come out when I fed them (only have flakes at the mo, going to get some frozen stuff tomorrow)
Danio 3: Being active, but at the very top of the tank. Is eating ok as well.
Danio 4 & 5: Being active all over the tank, but on their own, not schooling.
Danio 6: This one is being a bit of a vicious beast. Its staying right in the middle of the tank and whenever danio 4 or 5 gets near it it chases after them then returns to centre of tank. Its not biting them and the other two danios are in good shape, its just chasing them away from the centre.

Is this normal? I've read in another post that one fish is normally head of a school, so is danio 6 just asserting his authority and peace will break out soon?

Not sure on the sexes of them. The one at the top of the tank does seem to be quite a bit longer then the others, but not any fatter. Could this one be a female?

Have carried out water tests and all are at good levels (no ammonia/nitrite, minimal nitrate), no plants.

I may be worrying about nothing, but am new to the hobby so would appreciate your advice.

K

 

[Thirdfloor]

I have a few zebra danios myself, I can tell the sex two ways: first and easiest, the females have a silvery sheen to them whereas the males are more gold/yellow. Also the male has bigger and more yellow looking pectoral (?) fins than the females. Also, the females tend to be a bit more round-bellied. Hope that helps.

 

[korok]

OK, update already.

After finishing my last post, I went to check on the tank and danio 3 (at the top of the tank) has died

Now the two that were hiding have come out and the remaining five are schooling again.

Just carried out another water test and all is OK.

All a bit wierd

 

[korok]

I have a few zebra danios myself, I can tell the sex two ways: first and easiest, the females have a silvery sheen to them whereas the males are more gold/yellow. Also the male has bigger and more yellow looking pectoral (?) fins than the females. Also, the females tend to be a bit more round-bellied. Hope that helps.

Thanks for that. Three of the remaining five have distinctly rounder bellies than the other two so I assume have three females, two males. Can't really see much difference in their sheen, could just be my dodgy eyesight though.

K

 

[mark.w.jones]

Did you fishless cycle using the add & wait method from the link in my sig?

 

[korok]

Did you fishless cycle using the add & wait method from the link in my sig?

Yes I did. Test I did a little while ago shows ammonia and nitrite are zero, nitrate is at 20ppm.