Why wont my fish school?

my emperor tetra was acting really wierd and i thought he was just lonely cause he was alone (inside his species) so i got him some friends.. i put 4 emperors in 15.5 gallon tank and he started acting even weirder i thought maybe he was sick so i quarantined him and he started acting fine i left him by himself for a couple days no freek outs or acting up of anysort so i put him back with the other fish ( both the tanks are on the same filtration system and have the same ph) and he started freaking out again someone told me he had ich with the only symptom being his rapid movement so i treated all my fish i dont know how long its supposed to take for the treatment to work only that i should treat them for a week... never the less he just kept acting wierd so i removed him from the tank and put him by himself and now hes acting fine again. does he actually like being alone? i thought these fish thrived in schools can someone please help me... thank you