dying fish

Question
Hello,

I have had my aquarium for a very long time, almost 6 years. A few years back I added two rainbow sharks to my community. They did very well and were very active with each other and the others. Yesterday, they both died at the same time. They were acting a little strange the day before. I took them out and flushed them :( Today when I woke up and went to give everyone their breakfast I noticed I had lost three others. I think they were all my boshas (sp?) Nothing about the water is off, the pH, temperature, everything is normal. I don't understand and I'm really really sad that my fishes are dying.
I have a dinosaur eel that has never been a problem before, could he be killing my fish?? He swallowed whole some little yellow fish that I got a while ago, but he never killed them and left their bodies, he always ate them!!
Please help me! I am not going to have any of my fishes left!

Answer
Hi Lindzay,
Im sorry to hear the bad news about your fish :(.  If there are fish dieing at the same time, especially back to back with 2 days.  I would suspect water quality.  When you said nothing about the water is off, did you test the Ammonia, NitrIte and NitrAtes?  If these levels get too high, it is definitely fatal.  Have you been doing water changes?  A good rule of thumb is to do 15%-20% water changes a week to keep the the nitrogen levels in check.  Ph and Temperature are usually not the cause of death unless it is in the extreme zones.  

Hope I helped, let me know if you have other questions.
-Matt-