It sounds as if the most probable cause for the yellowing leaves is over-watering! Sorry, but more house plants are killed by this than anything else. Especially in the Winter because plants do not grow very much, if at all, & only feed during active growth periods i.e. in the Spring & Summer. Your home may not be as warm/dry as BFP's or you may have put plants in a cold draught. The best way to decide is to up end the plant & tap the pot off. You may see signs (or smells) of rotting roots & soggy compost - definately over-watering. You can replace the pot & keep plant dryer, warmer & lighter until it has dried out a bit & you might be able to save it. My rule is to touch the top of the compost - if it feels dry & doesn't stick to your finger then water - otherwise don't. This applies unless a plant label says 'keep moist'. However most plants do not like 'wet feet' & should not be left standing in water for more than overnight. Just tip any extra water away after half an hour. If roots have rotted I'm afraid it's the dustbin.