Hi Carol,
This happens to me all the time. I think its just the natural course of things. Maybe they eat each other, and only the strong survive? Would your carp eat them even if they are larger? You could possibly make a smaller little watergarden so that they could grow up into frogs/toads. Do carp eat full grown ones?

you poisoned them

I don't think so as there are no bodies left behind just decreasing numbers. They ate the food as soon as I put it in and seemed to enjoy it.

You didn't poison them Carol.