You can use marbles or river rock (dollar store) in the bottom. And don't pack them too tight. Little beaks and bills need to get in and drink and for the duck, to clean her nostrils. I also wouldn't keep the light over the water. They need zones warm, not as warm... They'll move to regulate. With the light over the water, they have to get wet. If they're not playing in the water, it doesn't have to be warm.