Why does my fish keep staying in the same part of the tank all the time?

when i tap the outside of the tank it moves and can swim, but then after a couple of minutes it just swims back to the same place in the middle of the tank and doesnt move? its a minnow, i have 3, and the other 2 tend to stick together and are fine swimming around, its just that one that is on its own and stays in the same place. i have had them all for 3 days and they are in a 10 gal tank with a heater and filter, i feed them every other day