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Hey guys.
I have a newly cycled (established for about 1.5 months) planted 20 gallon tank.
I got 7 Zebra Danios. They all look healthy but 5 of them are all together but 2 or so are almost always apart from the group. I have seen all seven fish school, but only a few times. Sometimes one of the Zebra Danios just sits on the ground for a few mins but gets right up just does not school.
Why is this?
THANKS!
 

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This is part of the reason that a school may be defined as a minimum of 5 fish, but I really feel that you need a dozen or more. There will always be one (or more) who refuse to be part of the crowd for whatever reason. There certainly could be health issues, so definitely watch the fish and see if anything else shows up. Perhaps the others are picking on the one more than usual.

I have 5 Mollies, and one of them is always apart from the other 4. Now Mollies are not schooling the same way that Danios are, just pointing out that there is 'One in (or out) of every crowd'.
 

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If they are the only fish in the tank, that could be another reason besides illness. Some fish tend not to school if they don't feel "threatened" by the presence of a predator.
Was about to say this.

Throw a larger, (hopefully) not super aggressive fish in there. My rummies school much better since I added a dwarf gourami.
 

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Zebra Danios are not schooling fish. They a shoaling fish. They only school when stressed otherwise you'll just see two or three chasing each other around randomly. They are very playful but drift all over the tank when they are not stressed. I'd actually be more concerned about the 5 that are schooling all the time. Also, when i had them, they never stayed in one place so I would be worried about the one that sits on the ground. They are super active and never stay still when happy.
 
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