Okay so this is my first time at owning fish and maintaining an aquarium, so please bear with me.
I bought the Aquarium (it's not too big.. only 5 gallons.. and yeah now I know i should have gone bigger after reading the stuff on here

), and bought 2 Tetras with the tank (the tetras have this totally amazing black dot on their red tails that really stands out). After a few days or so, and after seeing the tank cloud up and return to it's clear state, I bought 3 more fish:
2 Zebrafish and 1 Otto cat
The next day, I saw that one of my Zebras had it's fins and tails ripped off, and though alive, could barely move. It looked like it was in a horrible fight.
I moved both Zebras into separate tanks, and it looks like the sick and injured Zebra is doing better (i do not have them together btw)
My questions is why is the zebrafish so injured? I highly doubt he jumped into the filter, or that it hurt him.
I heard that Zebras need to be in schools of 5 or more, or else they attack each other. If this is true, then should i go get more, or should I return them for more tetras? I am afraid if I get more Zebras, they will gang up on the tetras, and I really love the look of the tetras.
Also, should I get Algae wafers (i think thats what they are called) for my otto cat? he is the only one in the tank, and I do not know if I should worry about him or not. he seems very healthy and is very shy (always hiding in the fake foliage.. and LOL wow I just looked up and saw him actually swimming outside of it for the first time haha).
Thanks for reading this everyone! hope you all can give me an idea on what to do.. =)