Fertility & Hatchability

I have a five year old pair of Dewlap Toulouse. To date they've only hatched two eggs. Both goslings were found dead. I think she stepped on them. Poor things were flat as can be. Other eggs I've taken and checked for fertility, but it's been more than sparse. I realize they need deep water to mate in. This time of year in Maine their pond is frozen so several times a week I fill a baby pool for them. I've never seen them mate in it. I expect her first egg of the year any day. It's abnormally warm this year. This could be helpful as I feel like the eggs sometimes chill before I can collect them. They are eating Poulin Layer pellets.

Anywho, any suggestions for food modification (or anything else) that might improve fertility/hatchability? Brewers yeast? Thanks.