is this yellowing only on the older fan leaves?your plant looks as if it has more than one problem.firs it looks to be over watered,because of the droopy look.there are numbers of things that could cause your leaves to turn yellow from bottom up,overwatering is one because it affects the roots.its drowning the roots which they are unable to get oxygen which inables the plant to uptake nutrients.may cause root rot.flushing and re potting would be my last option,which both of these are stress on plants.afther less watering and fertilizing,i would start with the simple solutions first ,such as correcting humidity,temperature,ventilation,pest,co2,viruses,ph.if that fails then try foilage feeding first,add too watering root stimulator,next watering epsom salt,next watering peroxide,add tiny egg shells on soil then if all else fails then i would flush and last re pot.most of the time if yellow only at the bottom its something going on in the soil.either to much or too little of something.good luck