Congo Tetras

Hi all I have 3 Congo Tetras I have had them for 2 years now they are with a community of tropical fish Discus etc in a 5ft tank but what I see with them now the 3 of them are facing the back of the tank and at the top of the water where before they were always swimming side to side across the length of the tank they don't seem to eat now to. All my Tests of the water are Normal.
Also all the other fish in the tank are great.
Can any one tell me what could be wrong if anything.
           Thanking you all Peter.

The small shoal size was my immediate thought too - our site recommends 5+ but, generally, the more, the better, and there is specific mention of the fish being easily frightened if in insufficient numbers [http://www.seriouslyfish.com/search/congo+tetra and http://www.seriouslyfish.com/search/congo+tetra].

Have some of the shoal died recently which might have made them more nervous? Might any of the others be fin-nipping them? Fish behaviour does change over time, so it's altogether possible that they may have co-habited well in the past but some have taken control in the tank and are intimidating the congos.

If you had space to at least double the numbers, and so that the shoal is/remains mixed-sex (as per advice in links above), hopefully that may make a considerable difference.