Why type of filter, decorations, plants (if any), other aeration is there in your tank? Does the tank have gravel and what kind is it?

What is your water source? How often do you do partial water changes and what do you do to prepare the water for the tank?

On the dead fish, were there any marks? Did their behavior or shape change any when they took sick? Were there any creatures hanging onto those fish or sticking out of them? Was they eating, breathing and evacuating (leaving feces) normally before they expired?

As for the algae eater, are they leaving the fish alone? Please see my first response to "Can't get rid of "aqua bugs!!"