Fungal problems are some of the most persistant issues facing growers. Even indoors, a variety of fungal organisms can affect your plants, ranging from common problems like anthracnose to opportunistic infections that attack your plants after a pest has weakened the plant. Chances are, if your plants start to suffer from unusual spotting or funny colored growths, the problem is a fungus.
Outside, growers use a variety of antifungals to control problems, including copper and sulfur. You can use these indoors also, providing you carefully follow the label instructions. However, if you prefer an even gentler solution, try using baking soda. Baking soda (sodium bicarbonate) is an effective fungal preventive agent and can even kill some established forms of fungus. Research has shown it's effective against some kinds of black spot and powdery mildew. Best of all, baking soda is completely non-toxic for animals.
A typical baking soda spray can be made by dissolving 1 teaspoon of baking soda per quart of water. You can add insecticidal soap or liquid soap (such as Ivory, just don't use a detergent) to help the solution spread and stick to the leaves. Spray the plant completely, upper and lower leaves, and let it dry. Repeat the application as necessary to control the fungal problem. If the problem continues despite repeated application of baking soda, consider a stronger anti-fungal.