We don't have much luck with houseplants. Our latest victims are a philodendron and a small diffenbachia. The philodendron's doing ok, but the diffenbachia is suddenly off its feed this week. We repotted it into a slightly larger terracotta pot when we got it, and we've been watering it every three days or so. The leaves started looking kind of sickly after a week (pale, drooping), but now it's dying for reals. Leaves are wilted, not getting any water up the stems, and there are tiny black bugs crawling on the leaves & stems.
What'd we do wrong?