Cute little guy! I love them at that size.
My IT is more comfortable in a tank that has a substrate. I had him in a bare tank for a couple weeks and he just didn't like it. Mine is stable 99% of the time. Give yours some decorations and gravel and he should become more stable in no time.
Growth rate depends on how well he eats. Mine grew from 1.5" to over 8" in three years. I've heard of faster growth and slower growth. Obviously the larger they get the slower the growth too.
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