what kind are they? how long have you had the aquarium set up?

almost 1 year rummey noses

Most small tetras do well in groups of 8 or more. I find IME they usually school at night or when the aquarium lights are off. They usually stay in schools when there is a potential threat, like bigger fish. Not that the fish will necessarily eat them, but it is potentially a threat. When they feel more "secure" they tend to spread more or pair.

ok, so you hvae had them for a year and you just bought some replacements, right? and these have also died, right? all at once?

i never had these fish so i don't know if they are sensitive or prone to anything.

My experience with them is the same as Woeisme is saying. Once they are comfortable and feeling secure they don't seem to shoal as often. They got about their business until they feel threatened or I turn the lights off. When I check them in the morning I usually find that they are together, I guess they rest that way for the security of it.