Hi some advice please i have aneon tetra that keeps hiding behind the filter/heater is this normal ?

hi all i have 2 neon tetras which you would think they would stay together but 1 on them keeps hiding behind the filter/heater box is this normal y? is it hiding i do have 2 balla sharks in (silver sharks) but they just keep themselfs to there selfs plus the other neon seems to hang vertical alot nera the heater/filter box any advice plz