[{_id:{"$oid":"5a934e000102030405000000"},aquarium:[{name:"Aqua A",fish:[{name:"Nemo",color:"Orange"},]},{name:"Aqua B",fish:[{name:"Goldfish",color:"Gold"}]}]},{_id:{"$oid":"5a934e000102030405000001"},aquarium:[{name:"Aqua C",fish:[{name:"Crab",color:"Red"}]},{name:"Aqua D",fish:[{name:"Prawn",color:"Gray"}]}]}]

db.collection.update({"_id":ObjectId("5a934e000102030405000000"),"aquarium.name":"Aqua A"},{"$push":{"aquarium.$.fish":{name:"Tortoise",color:"Green"}}})