I wouldn't assume it would stay smaller, nor eal way to know that. And even if it did it would be too big for a 75. But at 3" you would have a some time in a 75. They grow pretty quickly when small, depending on feeding schedule and water quality of course. I would say you'd probably have a year or so in a 75 at 3". But the growth rate is pretty variable. I'd move it up when it was a foot long.