I have two Poecilia Reticulata. They were fine up until about 4 days ago. One of them was sitting at the bottom of the tank, not swimming low but just sitting at the bottom. I feared it was dead, but at closer look it was still moving its fins. When i fed them, he would swim to the top and eat then go back to the bottom. I thought that maybe there was something wrong with the water so i proceeded with a normal 1/3 water change and cleaning just a little earlier than normal. The other fish seemed happy and was swimming all over the tank after the cleaning but the same one was sitting at the bottom again. Only occasionally do I see it move now. Is there something wrong, something i can change, or is it just dying on me?