Unknown Fish (solved)

Hi Sam 

They do look like pencilfish, the most common one in shops is Nannostomus beckfordi, the golden, or Beckford's, pencilfish. I have just turned around and compared your photos to my fish and they do look like my females. Male pencilfish have more red on them than females, especially once they've settled in the tank. If they are new, any males you have might not have coloured up yet.

If they are golden pencilfish they are one of the more tolerant species of pencilfish. But just to warn you not to use API's melafix or pimafix with them in the tank. I did once and they all died within an hour 

If you have room, I'd get a few more than to make them up to 6. I have 11 (bought 12 but one was deformed and didn't live long) and they behave well as a group.
However many you do get, try and work out of you have males or females first. Besides being a bit more colourful, males tend to have white tips to their pelvic fins (pair underneath the body) and the anal fins are slightly different - the rear edge is straight in females and curved in males. It is better to have more females than males unless you have lots of tallish plants.
Males will also 'spar' with each other. This takes the form of them swimming side by side, almost touching, and rubbing their bodies against each other. I have a pair of deeply coloured males who have done this almost since I got them and another pair of much lighter coloured males that have only just started doing it. Don't worry if yours do this.