Over the last couple weeks, the leaves of my Clivia have been yellowing at the middles and ends -- I'll post pics rather than try to describe. I've had damage like this happen before, but it was usually pretty slow and mainly on older leaves of the main plant. This is going fast, for a plant, and the worst of it seems to be on the mature leaves of the 4-5 "pups" that the Clivia has produced in the last couple years.

Background: The plant is about 13 years old, just a plain Clivia miniata. It's been in this pot maybe 2-3 years? I give it a balanced fertilizer once in a while, but I'm neglectful about feeding. I let the water sit for a day or so before giving it to the plant, and I don't let water sit in the saucer -- I have to swab it out because this 14-inch pot is so heavy. (I suspect that I may be overwatering; even though I run a finger down into the top of the soil to check before I water, maybe it's hard to tell with a pot this big.) I forget what kind of soil I potted it in. It sits about 6 feet in from a big west-facing window, full sun. I live in the desert Southwest, Zone 7-ish.

Any advice would be greatly appreciated!!!