I have found two turtles at my pond in the last several months that appeared sick. The most recent one had a very dirty looking shell hard to see any color and his eyes seemed covered with mucus given it the appearance of being blind. The bottom of the shell was clean and visible color. The other turtle was similar in that the top of the shell was dirty with no color and it had very small bubbles coming from its mouth and nose. I use copper sulfate in my pond to control algae. Is this making the turtles sick?