iridescent shark 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
Dear Priya,
I am afraid that your water quality may not be very good. Even though you said you got it tested. Sometimes petstores are wrong with when they say the water is "healthy" some people consider traces of ammonia and nitrite to be "healthy" when really you should never have ammonia or nitrite in your aquarium water since it is literally a poison to your beloved fish. An uncycled or unestablished aquarium can have ammonia and nitrite traces and sometimes even spikes which can certainly cause great fishloss. The loss of your Angelfish really makes me believe something is wrong--angelfish are very sensitive to ammonia and nitrite. And this could be why your Iridescent sharks are behaving this way too.

If your 30gal aquarium is only a few weeks old it is likely still cycling which means the beneficial bacteria are not yet established in sufficient number to remove all the ammmonia and nitrite. The best thing to do is frequent water changes. Make sure you have these things on hand for your water changes-
*Tank only 5gal bucket and aquarium siphon hose
*Bottle of water conditioner (stress coat, amquel ect..)
*Simple aquarium therometor
*1-2 gallon pitcher of aquarium only bucket (for filling)

It's best while your aquarium is cycling to do very frequent water changes to dilute the resulting high ammonia and nitrite levels. It would really be best if you had a aquarium test kit so you can know your ammonia, nitrite, and nitrate levels yourself. Testing daily is also important.
~Whenever ammonia or nitrite goes above safe bounds, an immediate 50% water change is greatly needed! Remember to insure the replacement water is equal in temperature to that of your aquarium and insure it is dechlorinated properly everytime.

Unfortunately your current fish are far from "beginners fish" the bala sharks have the potential (and do) grow large, sometimes up to 10 inches. And the Iridescent sharks can easily grow 18 inches and larger. Angelfish tend to be very sensitive to water quality and are best to keep after you have had some good experience at keeping fish.
Not to say you must get rid of your current fish. But it may be better off for you to stick to true easy fish than to have to deal with very large and sensitive fish.

If you'd like, here is a list of good hardy fish I have found in my experience-
*Black skirt tetras
*Lemon Tetras
*Zebra Danios
*Leopard and Giant danios
*Platies
*Swordtails
*Small barbs- Cherry, Rosy, and gold barbs
*Corydoras catfish
*Dwarf Gourami

And there are are lots more hardy and small beginner fish. but remember no fish can handle polluted water. But with plenty of frequent water changes you can easily make it through with the cycling and soon you will have an established and healthy tank with no more worry of ammonia or nitrite problems.

I think it may be best to do water changes on your aquarium now. 30-50% is a great place to start and at least 3 times a week until your tank is for sure established.

I really hope this helps. Only my very best wishes,
Karen~