Fish Biting each other during feeding

Question
I am noticing weird behavior in my fish tank and I do not know if it is normal or not.

I have a 55 gallon freshwater fishtank.  It has excellent filtration as I have an eheim external filter capable of handling a 300+ gallon tank.  The water is crystal clear.  I have several air stones running along the back and the sides of the tank.  I have a pump also creating an additional current.  Lastly, there is algae that is has grown on the wood in the tank that rocks gently back and forth with the current (a good sign).  Note:  the algae is green, not free floating at all, and is under control as in, it is not growing over everything, just key areas.

Occupants:  1 Black Shark about 8", 2 Bala Sharks about 7", 1 Rainbow Shark about 3", 1 Red Tail about 2", a fake zebra pleco (fake as in it is not a true zebra pleco).

Here is the problem:  During feeding, and only during feeding, the three largest sharks ( the black shark and both balas ) will start biting / nipping at eachother.  I realize this is normal during feeding as they want the food to themselves but this goes beyond that.  They actually seem to lose interest in the food and focus on each other and they bite more than just fins, they actually focus on the body.  What is also weird is that they don't seem to mind and they keep "taking turns" on eachother if that makes sense.  There are risidual bite marks on their bodies and fortunately, all the fish look healthy otherwise - no fungus or disease.  

I have had this tank set up for over two years with the same fish for most of that time.  I have never noticed this behavior until recently.  I have two theories going.  First, maybe they are starting to get too large for this tank?  Although they are completely peaceful with eachother when they are not feeding.  The other theory is that it has something to do with the temperature of the tank.  I try to keep it about 78 degrees but with the recent heat spells, it has pushed my tank up to the high 80s.  I had to unplug the heater to get it to finally come down.  This only occurred for a couple days and the temperature is normal again.  I am not sure if this is an issue or not but I thought I would mention it just the same.  I do not think that this is the main cause as they were behaving this way prior to the temperature problem.

I asked a guy at a fish store about it and he said it could be that they are stressed about something, producing extra slime on their slime coat, and that they are eating it off eachother.  

Any thoughts are greatly appreciated.  I just want to know if this is normal or if I should be worried.

Thanks!

Answer
I am not sure why the fish person would suggest the fish were eating the slim coat. Yes, when under stress the fish produce extra stress coat but I have never heard of the eating it, especially off each other.

I think you have done a great job at diagnosing the problem yourself. I was thinking as I read that the fish were perhaps getting too big for the the tank even though they have lived peacefully before and are considered peaceful fish and you have a large tank. I have had several bala sharks and the same thing always happens: once they get around 6 inches the begin to change and become more aggressive. Basically fish that once were living in harmony now have turned mean and begin to beat up on one or all of their mates. I highly doubt that the temperature is the problem and I cannot think of another issue. Aggression due to a growing fish is also very likely because it has begun to happen only during feeding. This is showing that they are desiring more room and feeding just agrivates them enough for them to take it out on each other. Now I do think it is unique that they all look healthy and are doing well. Usually the fish will target one fish and soon the fish will be huddled in a corner and look sick. Your do not seem to be doing that.

Bottom line, the very likely cause of the aggression is growing fish in a smaller tank. The only remedy of course is a bigger tank or seperate tanks for each. I hope this helps and feel free to ask anything else you need.