iridescent sharks sick?

Question
I have a 30 gallon tank with 2 bala sharks, 2 iridescent sharks, an angel fish, and 4 other tiny fish.  I'm not sure exactly what type of fish the other 4 are.  I beleive they are some type of barbs or something.  Hopefully the fact that I don't know exactly what type of fish those other ones are won't matter in your answer attempt to my question.  We as a family are beginners to the whole fish thing.  We were told to start of with "easy" fish, so those were the ones we got.  Now, the problem is that the iridescent sharks were a little anti-social from the beginning.  They seemed to stay towards the bottom of the tank until fed inthe morning.  About a month later, they began to get more active during the day and would swim around much more.  Now in between now and the time they were active, we had gotten two other very small zerba fish (i think).  Those little ones were orange with black stripes.  All four of the sharks ganged up on the little zebras and killed them.  Then one of the 2 angel fish somehow died.  After the death of the second angel fish, the iridescent sharks have now started to settle at the bottom of the tank and hide themselves.  Now I did read that they are shy and that they do like to settle at the bottom of the tank, but I'm afraid that something might be wrong with them.  Also, I did read that they are very easily startled and that they run into the sides of the tank when they get scared sometimes.  That did happen a couple of times and I don't know whether that contributed to the fish acting that way.  Please help me figure this out.  I will go back to the pet shop and try to figure out what type of fish the other 4 are in the mean time.  Finally, I did go and get my water checked to see that it was healthy, and it was.  Again, I greatly appreciate your help.  Thank you.


Answer
As a general rule yes that is how their behavior is, they stay more towards the bottom. Not all fish behave the same according to the the rule of how they are suppose to act exspecially in numbers. Your angelfish have those nice long fins which make easy targets for other fish to nip at over time can cause stress to the fish and kill it. Little fish can occasionally be interpretid as a snack. Do you have any cave structures on the floor of your tank for them to hide in? If not I would get some and add them in as well rearrange any plants in the tank. Adding the caves structures will give them a place to hide and feel safer being they are a more shy fish by nature. Rearranging the plants and other decor will break up any established territories that have been claimed by any of the fish. Their first priority will restablishing their area in the tank. This should help you out. I think their behavior isn't out of aggression as it fear. You also made a good choice in having your water tested prior to seeking additional help....most people do not realize the importance of water testing. Keep up the great work and happy fish keeping!