Is Quick Start Killing My Fish?

[eaglesaquarium]

So now I have two zebras in each tank and they are doing great except attacking each other constantly.  One just lost half his tail last night.  How do you keep them at peace?

 
More of them, and in a bigger tank.  
 
 
I'd add more and deal with the ammonia issue.  The colony should grow fairly quickly from this point now.   As Rainbow says temp does play a role, but I'd worry more about the dangerous free ammonia.
 
https/www.hamzasreef.com/Contents/Calculators/FreeAmmonia.php
 
Use this calculator to find out how much of the toxic ammonia is present.  Keep it under 0.005ppm.  But you'll notice that 0.5ppm 'ammonia' at 77 degrees F and 7.0 pH is roughly half of the 'dangerous level.  If it gets too close to that level do a water change to lower it.

 

[wake]

Akasha is right. Each tank has a colony or two of bacteria that are big enough to process the ammonia produced by the fish, and no more. If you add more fish, more ammonia will be produced, so your bacterial colonies are insufficient. With an immature tank, the bacterial colonies will take a while to grow, so you will have surplus ammonia in the water.
 
The usual advice is not to increase the bioload (ie the ammonia produced by the fish) by more than 50% - since you only have one fish, and want to add more of the same, you can only add a minimum of 100% at the moment. You will need to watch the ammonia and nitrite levels for a while. Then you can add another fish, which will be the 50%. And then another, which will be less than 50%. Then you could add 2 more, which would be 50%, which would take you to 6.

How long would you suggest I wait before addition of more and more fish, week or two?  I added one more fish each and ammonia and nitrite levels are all still at 0