Help! My ducks are drowning!?

My four ducks are almost a year old and we just moved them from there winter shed into there old out door pen (large dog run) before winter that is were they lived and had no problem with there stock tank swim area. Yesterday (the day after we moved them into the outdoor pen) when we went to care for the duck one of my Harlequins was dead and seemed to have drowned we initially thought that the ducks got to excited about the water tank and drowned her since we have two pekins there as well they take up a lot of space in the tank. Well today we went again to take care of them and one of the pekins was dead! It looks like she died the same way. We took the tank out but do you think it could have been something else that caused them to die besides the water? Also the stock tank is at ground level so the ducks can get in and easily. Thanks in advance.