"Could the death of lilypads be caused by the algaecide? The label
claimed it was safe! "

O shoot! I only wanted to subdue the algae, not the other water-plants. Good thing I did not throw it in yet.
Shac Ponder. Website gives no info, just that it is safe for fish.

I have a bale of barley straw soaking in there already, and that does NOT harm the waterlily, but does not do all that much for algae that cover all the waterplants.