Trouble In The Tank

[OLLYC]

I added my first fish to my 56 litre tank a week ago. I added 3 leopard and 3 Zebra Danio's.
3 days ago, one of the zebra's was looking very unwell and though he appeared to have energy, he seemed unable to swim and eventually died.
Today I discovered another of my zebra's dead in one of the plants. It's skin had turned completely pale and I think I found a large part of it's skin close by (the leopards eventually removed it for me).
This is worrying me and I am going to do a water test tomorrow. I have also noticed that the leopards seem to be very aggresive and harrass mainly the Zebra's but also each other.
Could it be due to the fish attacking each other? Overfeeding?

 

[Fatty]

I had 5 Zebras once, one was a real bully and ended up killing most of the others until I re-homed him, maybe you just have one bad one??

 

[Davo86]

How long has the tank been set up?
Erratic aggressive behaviour can be a sign that there is a water problem, Was the tank cycled?

 

[waterdrop]

Yes, when did your fishless cycle finish and how many days had you showed zero ammonia and nitrites before the finish?

 

[Miss Wiggle]

considering they are the first fish added to the tank i am very suspicious that the tank is not cycled, have a read of the link in my sig 'whats cycling' which should explain it all to you.