Did you take enough time to acclimate the HSB before letting them loose in the pond?

I would wait on getting more HSB until you see how much more mortality you have with them, and if possible, the cause of the excessive morts.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."