I happen to have both in my 115g tank, and they do not socialize cross-species. The marble hatchetfish all interact from time to time, and the silver hatchetfish interact from time to time. But never across species. They don't fight, they are quite compatible, but you need a decent sized group of each species.
And hatchets are always better with more. With marbles, I would not have less than 9 or 10. The Silver are fine with 7 or more. But in your 38g, which I will assume is probably 3 feet in length max, I would go with the smaller Marble Hatchetfish, and get a dozen.
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