One of my 3 (3 week old) pekin ducklings seems to be laying more then standing. It can get around if it wants to, but seems to maybe have weak legs. It will only stand for a very short time. I am new to raising ducks and need advice on what to do. Should I use brewers yeast or niacin? Which is less expensive? Where do I find it? How much do I give? Should all three of my ducklings have it or just the one with the weak legs?