Debbie, I use compost and Osmocote, and my fish, cats, dog,
wild birds & wild animals that drink from the pond are all still alive.
Water is gin clear, but then again, I only use Osmocote or Multicote.
I never use pond tabs or any similar 'tab' fertilizer.
Just Osmocote once in the spring when I repot, and that is it for the season.
Simple, cost efficient, effective ...
AND EASY!