If the eggs lay flat at the bottom of the water they are fresh, if the egg stands upright on the bottom then they are getting old. If they float, throw them out, they are too old. This is a very reliable test, because as an egg ages, the air bubble in the egg grows larger.

Do fertile duck eggs float?

Fertile duck eggs should sink in water, unless they are old and rotten.

How can you tell when duck eggs are fresh?

If it floats, it is rotten. Simple! You should store the eggs broad end up pointy end down, in a cool place. It is not necessary to keep them in the fridge except in very hot weather.