Answer: Sounds as though you are doing everything right for your plant. If it is still producing healthy new growth and is flowering, I suspect that the yellowing leaves are normal. Hibiscus retains its leaves for quite some time but after a few months they get old and inefficient. The plant regularly sheds these old leaves in favor of producing healthy new leaves. If you don't prune your plant it can get leggy (bare on the lower branches). So to compensate for the oldest, innermost foliage drop, prune the stems back. This will encourage new growth towards the middle of the plant and your hibiscus will look full and lush.
Best wishes with your hibiscus!
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