Personally I would use them
If anything I would break them up a little and place a couple of small bits around the roots of your plants, use only with swords, crypts and lotus things that are heavy root feeders dont place under the roots of stem plants
I have used manutec fern spikes, manutec pond?, osmocote native(about 8 pieces frozen in an ice cube trays) and dino dung (blood and bone mixed with clay)
There should not be anything in them that can harm fish as long as you dont go overboard and use a little common sense
As for pond use rember there are small ponds as well as big ponds
Ray