They could also need more light and a warmer location. If they have been blooming and have stopped, they may be done for the season. They will bloom the longest where they get strong light, are fed regularly and are kept evenly moist and warm. This can be the result of drying out or being kept too warm. This is from being kept too cold.
Dark patches can be the result of leaf shine or watering with cold water.
If the leaves turn brown and dry up, check to see if the plant is being kept too wet or too dry. Pick off the brown leaves and watch closely. If they are kept too wet or planted too deeply, gerber daisies will develop a crown rot that is almost always fatal. This can be avoided by using a well-drained soil, monitoring the water closely and making sure your daisy is planted a little bit higher than average. Occasionally, the leaves will develop a white mildew.