bent spine

Hey I'm new here :) I've got a +/- 26 (?) gallon tank (100 litre) with 6 Zebra Danio's, 5 guppies and about 15 shrimps (hard to count, they keep hiding). I'm kind of a noob, so I'm not sure if its normal, but at first my zebra's were very lively. swimming al over the place and almost impossible...