Are my fish eating each other?

Question
I started my community tank at the beginning of this year, in hope of gaining some knowlege of the aquatic area for a school project.  I began my 29 gallon tank with three danios:  one short finned leopard danio and two zebra danios.  One of the Zebra Danios was larger than the other two, and would chase them around my tank.  After my nitrogen cycle was complete, I got two long finned Leopard Danios and another Zebra Danio that was larger than my others (in hope that the new bigger one would "create peace" with the others because of it's size).  I also bought 4 Black Skirted Tetras to add some variety to my slowly growing community tank.  When I was acclimating my new fish however, I noticed that the smaller Black Skirt Tetra was missing it's left eye, with no signs of it anywhere.  When I metioned it to my dad, he said it was probably just rough handling.  We didn't worry too much about it, and just accepted its handicap.  So my Black Skirted Tetras seem to be doing fine, but I'm worried about my danios.

A few months ago (maybe around March/April) I noticed that my short finned Leopard Danio was missing its tail.  Where there used to be a Caudal fin, there was what seemed to be a red stump.  Worring, I isolated it in a jar while I went and bought a breeding tank.  We kept it in the breeding tank during the night, but it didn't last to morning.  We took the dead fish to PETsMART to ask about this sudden condition and came to the conclusion that one of the other fish had taken a bite out of this one, maybe because it was the only shortfinned leopard danio and therefore making it different.  We also figured that maybe my fish weren't getting enough of the needed nutrients and proteins, and so we bought a different kind of food.  They also noticed that the danio was red around the gills and may have had red gill disease, but they couldn't tell for sure because "fish usually become bloated after they die and may turn red around that area."

A couple weeks after the loss of my first fish, I noticed that one of the Long Finned Leopard Danios began to get torn-looking/ratted fins.  Suspecting that the other fish were trying to attack this one, I isolated it in the breeding cage I had earlier bought.  Isolating the fish seemed to help because it made a somewhat decent recovery.  I returned this fish to the tank the next time I cleaned my tank, and it and the other fish seemed to be doing good...until a couple weeks ago.

In the first part of June, I went on a school trip for four days, during which my parents were feeding my fish.  When I left, I recall seeing the familiar nine fish still swimming around (although I don't think I took a head count).  However, when I returned, I counted only eight.  I asked my parents if they had noticed any dead fish floating at the top, and they said they hadn't.  I didn't notice anything wrong with my tank except for the absence of one fish, so I just accepted the fact that it's body was probably eaten.

I am over the loss of my first two fish, but now another problem arises.  I have noticed that one of my Zebra Danios have few marks on its left side.  They appear to be bite marks and don't appear on my other fish.  Just today I observed that there are more than there were a few days ago.  I plan on isolating this fish in the breeding cage as soon as possible.

That's my story, but here's my questions.

Is it most likly that it's my other fish that keep eating these select few?  Or is there a certain disease that keeps killing them off?  If my fish are eating each other, why?  And how am I supposed to stop it?

I have a 29 gallon tank, and each of my fish are only supposed to take up 2 gallons.  With only eight fish total, that means that only 16 of my gallons are actually being used, leaving 13 extra gallons.  Is this large amount of extra space causing my "schooling" fish to become territorial?  Do I need to add more fish to fill up this extra space?  But I'm worried that these new fish will be much smaller than my 1 1/2 inch round Black skirt tetras and may be eaten in one bite.  If I was to add more fish, what kind would you suggest, and should I get more of the kind I have?

I feed my fish the same kind of food, and I feed them once every day, and twice every other day. (Example: Mon. once, Tues. Twice, Wed. once, Thurs. twice, once, twice, etc.)
Do I need to be feeding them different food, different amounts, or anything like that?

When I clean my tank or rearrange my tank decorations my fish school, but after that, they retreat to their own part of the tank, separate from each other.  Is this more reason to add other schooling fish, so that they are intimidated and stick with each other?

Please Help, I'm at such a loss for what to do.
Thank You.

Answer
Hello,

Wow! That was a long read! That's alright, the more detail the better.

Some of this does sound like vicious fish, but some doesn't. Fish lose eyes sometimes. Its common in guppies. That can't be helped and is usually not caused by fighting.

Ragged fins are caused by either fighting or fin rot. Because your fish had no fin, I'd say it sounds more like fin rot. You should try adding an antibacterial to your tank. Melafix is a good one.

It sounds like you are neither over or under feeding. Too few fish shouldn't cause territorial problems either, so both of those are good. You should monitor the levels of nitrites and ammonia in your tank, and if you aren't doing so already, add a denitrifier weekly so that the levels stay at a safe amount.

It is good to give your fish a varied diet. You should continue with dry foods as they are well balanced, but also give occasional freeze dried or frozen blood worms, brine shrimp or even mysis shrimp.

I hope this helps! If I missed anything, feel free to contact me again.

Good-luck,
From Stephanie