Cycling - Zebra danios

Question
I'm cycling a 5.5 gallon tank using two zebra danios. They've been in the tank for four days, and though they both dart around the tank, one  seems to do all the chasing and occasionally nips the other.

Today the quieter zebra danio is resting in the corner of the tank on the gravel. It's alive, but seemingly not doing too well (the other will come around and nip it, and it will barely respond). The other is darting around as usual.

Both zebra danios are also much fatter than when I purchased them. I don't believe that I've overfed them. Is this a water-quality issue?

And is the problem with the quieter zebra danio related to the the bullying, or is it also possibly a water quality issue?

How could I resolve this?

Thank you.

Answer
Hi Kiri
Hard to say exactly what it is, maybe a combo of all of the above lol.  Zebras are active fish, and normally do dart around.  They're also best kept in groups, but with your tank size, it's not recommended.  So, the bullying could be due to only 2 of them, or due to the tank size.  Or, it could be a male and female, with the male chasing the female around.  The swelling could be just from them eating or from poor water quality-which can lead to dropsy, or they could've been sickly before you bought them as well.  
And the bullying does stress fish out, as well as water quality.  So, you're pretty much right on everything, just it's hard to say what's causing what....if that makes sense :)  For the water quality though, you could test that with a test kit to make sure, which you should have on hand anyway to see how the cycle is progressing, and to monitor it after it's completed.  Really the only thing I can think of to resolve it, is to return the fish and finish the cycling with fishless cycling.  Basically, just get yourself a couple of cocktail shrimp, put them in a mesh filter media bag or pair of nylons with the leg cut off, knot it closed, and toss it in the tank and let it rot.  This will produce ammonia-starting the cycle.  

Good luck with your new tank!  Let me know if you have more questions.

Christy