Answer: Sounds like you're overwatering your rubber tree (ficus). When overwatered, Ficus doesn't seem to have the same "shut-off" valve to stop water from coming into the plant when it has had enough, the way other plants can. Instead, it will continue taking up water until the leaf cells burst. Often, the first thing you see is that the center vein and the area around it will become bright green, then blacken within about 24 hours with the death of the leaf.
To avoid this kind of problem, periodically check the soil surface of your Ficus. If it is damp 1/2 to 1 inch into the soil it doesn't need water. This can change in just 24 hours, especially with indoor heating in winter, so check again, until you get a sense of Your plant's water usage.
When you do water, water Thoroughly, allowing plenty of water to flow out of the bottom of the pot (removes accumulated mineral salts). Never "soak" your Ficus or allow it to stand in water. Remove ALL water in the drainage pan within 5 minutes. (I have read that some people use "15 minutes" as a guide, but if I did that, I'd have a dead Ficus on my hands in short order.) This can be difficult with very large trees, but you may be able to tilt the pot to aid in complete drainage.
Adjust your watering and your ficus should rebound. Hope so!
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