fish deaths

Question
Hi,

I have a 12 gallon tank that I set up 5 weeks ago.  Prior to this Wednesday, I had 3 male guppies, 3 female guppies, and 2 cory catfish.  On Wednesday, I added two female guppies.  They appeared to acclimate well.  On Friday night they were active and eating well.  On Saturday morning, one was dead and the other died hours later.  This morning, one of the corys and one female guppy was dead.  I checked my water quality and everything thing is fine. The temperature has been constant at 78 degrees.  I have notice no apparent physical abnormalities on any of the dead or live fish.  Help!

Answer
Hi Anne,
The problem is that you added too many fish at once.  When adding fish to a tank, you must do this very slowly, depending on the size of a tank.  For a 12 gallon tank, you add one fish, and wait one week, check your water, and if it is right, add 2 more, wait one week, check your water, and add 2 more, and so on.  25% water changes every week is a must.
Your water chemistry has surely changed.  There is no other reason for them to have died.  Check your ammonia, nitrite, and nitrate levels.  Your ammonia should be zero, your nitrites should be zero, and your nitrates should be low.  I am certain that your water chemistry is off, thus causing death to your fish.  I'm sorry this has happened, but it is a common mistake that nearly all beginners do.  
I would also like to mention that you cannot add more than 6 fish, and maybe 3 little bottom fish, such as corys, to your tank.  An overstocked tank will bring trouble in water quality.  We must follow the rules, and always keep our fish free from stress, and give them enough room to swim!
I hope this helps.
Lynda