Soil is only one factor by: Jacki Cammidge, Certified Horticulturist
There are a few things that are important with Aglaonema (or Chinese Evergreen, as it's commonly called). If they are grown in low light conditions, it's important to never overwater them, as they're prone to root rot.
Your plant is showing signs of Anthracnose, which is caused by the pathogen Colletotrichum, a bacteria. Typical symptoms are circular to oval brown leaf spots which have a yellow halo. Spots may become 2 inches in diameter and contain tiny black fungal fruiting structures.
These can be seen under a magnifying glass, but will only show after the plant has been infected a long time.
The appearance of this disease is usually after overhead watering, where the air temperature doesn't allow the plant to dry off quickly, allowing the pathogen to enter the leaf. There is no real cure, only prevention.
Water the soil surface only, not the leaves. And, if in low light conditions, the soil should dry out completely in between.
The leaves can be cut off, but as they are still doing their job, this is a last ditch effort to save a doomed plant, so you're not there yet. When the leaves fall off, get rid of them, don't let them stick around to cause more infection.
Give the plant more ventilation, such as a small fan, to allow the plant to dry out faster, and if possible, move it into a brighter situation so it gets more light.