My parents have been having problems with their pond I mean the pea green color,not the usual blanket algae. Even the water hyacinths weren't looking good, well yesterday all their fish were dead, I told them last week when I was over the fish were suffering from lack of o2 they put in another filter fountain. Anyone have any ideas on this? and what they can do to prevent this. Some of those fish were at least 10 yrs old.