Beginner Gardening: Help! What's wrong with my Calathea?, 1 by Ed22323

Ed22323 wrote:
Hi everyone,

I've had this plant about two years. It lives below an east-facing window so has plenty of daylight without being in direct sun. The room's got an average 50% humidity and the central heating doesn't go above 21 celcius, and is off at night and weekdays.

I throw in a glass of tapwater with a few drops of plant food every 7 days spring/summer and every 10 days in winter.

It has been happy until about a month ago - several strong stems with wide leaves. Of late it's started having yellow edges and brown bits to some leaves, and the stalks are splaying horribly - see picture!

Can anyone who knows this plant advise me?!!

Overwatering? Underwatering? Too much/too little plant food? Too hot or cold, not enough or too much direct sun?

My thanks in advance! :)