Recently my male guppy keep dying. I have lost four of them. What i notice is their tail become smaller and smaller, the tip of the tail finally become a point. The shape of their tail is like a triangle. And after a few days they died. This happened after i bought some yellow guppy and put them in the tank. But all the new yellow guppies are very good and healthy now. I dont know if it is related to them. Any body can help me out? Thank you.