What is this fish?

[guppy]

There are many species of pencil fish and the similar headstanders, very few pencilfish exceed 2"s when full grown, the striped headstander is unusual for a headstander because it seldom stands on it's head and has a forward pinting mouth like a pencil fish instead of an upward turned mouth like most headstanders. the uppermost stripe is also distinctive in the way it is split before the dorsal but joined behind it, the striped headstander is the only one I have seen like that. here are some pics I found at various sites of assorted pencil fish.

 

[turtall]

Our nslfs has a tank full of those black and white striped ones. What can you tell us about them?

 

[Waldo]

How much do you want for that one in the top picture? I think you mis ID'd it though.... looks more like a Pen Sp.... could be wrong though * aside *

 

[Xuzhu]

i was think is pencil fish too

 

[guppy]

Our nslfs has a tank full of those black and white striped ones. What can you tell us about them?

I did some checking and this is what I got,
the top pic is something silly I found on a grade school site that popped up during a search for pencil fish before switching my search to characins.

all four of these fish are from the genus nannostomus and are small peaceful omnivores that will do well on granules, flakes, and tubifex/blackworms, and brine shrimp. They are all schooling fish and should be kept in groups of 5 or more.

the top fish is N. digrammus (twinstriped pencilfish) 1 3/4" max

middle left is N. harrisoni (Harrison's pencilfish) 2 3/4" max

middle right is N. marginatus (Dwarf pencilfish) 1 1/4" max

and bottom is N. unifasciatus (one lined pencilfish) 2 1/4" max

There are lots more and are seldom labelled as anything other than pencilfish.

Odds are if it looks like a pencilfish but has a slightly more robust head and is over three inches long it is one of the many headstanders instead.