Try leaving some in overnight. I think your Plec will eventually take it. If you leave some in overnight, take it out in the morning as you don't want to foul up the water.
When I feed zucchinnii, I cut it in half lenght-wise and use a fishing line weight to weigh it down. The top of the weight where the fishing line would go through the loop is small enough to punch in the skin of the zucchinnii but fat enough not to pop back out. By cutting it length-wise, I punch the tip of the weight through the skin and when the zucchinnii falls to the bottom the weight causes the skin part to be on the bottom and the soft squash part of it is on top.