I think that if you say jazak Allah . you have to say what . like when you say god give you . you have to say e.g jazaka Allahu khairan or jazak Allahu Aljannah ( paradise ) and many other things sometimes I heared a sad people thay say jazak Allah just and it means like< Allah will judg you< so if you want to say it say with something you wish it for your brother or sister . I don't know this is my opinion . if anyone have any answer please tell us