How are my rasboras are isolating from my school?

I had 6 rasboras, one was near the filter isolated and finally dead, now another one still living is isolated near the filter.

What illness are they affected by?

Anonymous2016-07-24T09:51:33Z

There might be a larger fish that is bullying them and causing them to hide. Watch the other fish in the tank for a while to determine if this is the case especially at feeding time . Turn out the lights in the room and get the room as dark as you can to observe them. Of course you won't turn the aquarium light off!

noselessman2016-07-24T10:06:09Z

What size tank, how long has it been set up? what is your cleaning schedual/method? What are results of tests for ammonia, nitrite and nitrate? Is the temperature stable and at 78-79 degrees?