I think it is either a fungus or being over watered. Either way, the best solution is to make sure you use room temp or slightly warm water for the plant (cold water on the leaves can make it spot and yours looks a bit tropical), don't over water, and make sure the leaves have plenty of time to dry before it gets dark. These three things will help reduce chance of fungal infection.
If you can take a leaf in a plastic baggie to a nursery or home depot, the people there can help diagnose which fungus is attacking your plant and get you the correct treatment.