Sorry to hear that your maranta is having trouble!

High humidity and protection from strong sun are the essential ingredients for keeping this plant happy. They are tropical rainforest plants that grow low, under all the leafy protection of the tree canopy and do best when they get filtered light (a sheer curtain), warmth, and constant, high humidity of 70% or more. To get that level of humidity, you can try a wet pebble tray, humidifier or frequent misting with distilled or rain water and use a simple hygrometer to see if you are achieving enough water vapor around the plant. A terrarium environment is usually the most successful solution.

From October through February, reduce watering from constantly moist to letting the top of the soil dry, always using tepid water. If you treat your plant to a little taste of the tropics, it will reward you!

Courtesy of NYBG Plant Information Service