I suspect possibly too wet, let it get a little dry, but not bone dry between watering. Dig your finger at least half inch deep into pot to decide if wet, barely moist or dry. You want to wait to water for the barely moist. Check to see if it needs water daily, only water when it needs it. Indoors this might be every 3rd or 5th day. Notice the weight, or heft of the pot. Dry soil the pot will be light, wet soil is heavy. In a few weeks you will not need to stick your finger deep in the dirt, as you will know by the weight of the pot.
I would not treat for fungus. Fungicides are not "harmless" they stress the plant and can send a weak plant even further into decline.
I would just stay on top of your watering, letting it get to the barely damp before watering again. Then when summer arrives, put it outdoors to recover. This is called "limping my tropicals through winter" waiting for summer when they can wake up and grow.