When they are fertilizing or have recently done so if you can smell it then you can be assured a good dose has reached the pond .I'd play with the idea of a bog on the left / straight sI'd of the pond start at the waterfall so it will help the waterfall and bog being raised look more natural and not the volcano theme. let the bog discharge push the water toward your skinmer. Where you are having the green issue id make the bog as bid as I can the average is 1r/3 . I'd even try for a littlre more don't forget the bog can have some amazing flIwwrs and plants so while the cost may add up in the long run they are very cool.