The scenario you describe may not be the greatest for the swans. First, any cement substrate can cause bumblefoot ( a serious foot condition) or leg and foot injuries. Dog food is the wrong food as it contains too much protein. The swans need to be fed 1/2 poultry layer pellets mixed with 1/2 cracked corn. Lettuce should be provided to the swans to supplement their aquatic vegetation as these seems to be almost non-existent. No tubers is a good thing as these can be somewhat poisonous depending on the species of plant. Grass is a good grazing food and bushes and other plantlife need to be checked to ensure that nothing poisonous is being planted around the habitat. We hope this information is of benefit to you. The Regal Swan