I sat a 10 gallon tank up about two months ago and let it go through the cycle and everything. I got 6 black skirt tetras and 4 swordtails, I went to spend the night at ny sisters and when I came back 4 of the tetras had did and one just now died and the last one is about to die because it's on its side. I started checking them and 1 swordtail has lost hald its fin on one side and dont move it much. all 6 of the tetras had the tail fin mostly gone ive been changing the water out once a week and feeding them once a day. Can anyone tell me what happened? They all seemed fine two days ago.