Do you actually do it or do you just have a few carpenters in your family?
I mean you do realize that almost all wood will have moisture in it when new? Ever shot down a treated 2x4 when laying out the walls of a house on a slab? The amount of water is ridiculous.
Also an aquarium has weight distribution on the corners. Ever wonder why you can put a aquarium on four cinder blocks and be good to go? I mean unless you advise against that because you come from a family of concrete workers? Perhaps engineers?
Stands that are recommended for aquariums to keep the warranty - even up to 120g (probably more) - are made of 1/2" pine and staples... Not 4x4s and engineered beams.
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Douglas Fir is unique among all softwood species in that it is naturally dimensionally stable, having the ability to season well in position. Many builders prefer to cut, nail and fasten Douglas Fir in the "green" or unseasoned condition, allowing it to air dry during construction. There is a reason they keep it that wet... to keep shape from the mill.
I have lived in the Pacific Northwest all my life, and we have just a little bit of Douglas Fir here, lol. I've worked with it often, and it can be a very beautiful wood. We've made a few incredible entertainment centers from it. My concern is not the fir, it's the green part.
True green is 40% water content or more, and there is a really high chance of warping. It may be ok depending on how he builds and supports it, but I'm saying what it might dry out to could be pulling on itself in a way that makes the legs unstable (wobble, not level, more pressure on one board). The larger the stand, the more worried I'd be about it.
I may be the first in three generations to not do it for a living (mostly custom cabinets, doors, trim, etc), but that doesn't mean I haven't spent my time in the shop