Why are my baby bullfrogs showing up out of the pond dead?

I have a pond in my backyard (12 to 14 hundred gallons) with about 30 gold fish and a school of minnows. I also have water lilies and several other water plants with a bio filter, huge waterfall, and 3 UV lights. Last summer, I got a ton (75-100) tadpoles and they are now growing up to adult form. In the last week, I have found 6 in my yard and they are dead. I don't think birds are getting them because when I find them, they are not mangled or detached, they are whole. I have not used any kind of fertilizers on the grass or any other chemicals. I do have a lot of algae growing and am trying to get my pond water to clear up but I don't want to use any chemicals to do so. I am at a loss and hate to see them die. Any suggestions?