You can bolt steel or aluminum together and it will be just fine, you should design the stand so that your joints, rather wood or metal are loaded in compression rather than shear. In simple english that means that the weight of the tank pushes the joints together rather than trying to rip them apart. Fasteners should serve only to hold things together when you're moving it around and keep pieces from slipping off of each other. That's the idea anyway, some times there are situations where it's just not possible, I've found nuts and bolts to be remarkably strong, the best would be socket head cap screws (google knows what they look like if you don't) a 1/4-20 SHCS is rated for minimum 5730 pounds tensile strength, that's a lot of water on one bolt. Stay away from stainless fasteners, I know they look nice and it sounds like a good idea for a something intended to be used near water but they are a bad idea. Stainless fasteners are usually not graded and not rated for structural applications, many "stainless" fastners will rust as well.
Drywall or deck screws are not very strong, if you've got to use them make sure they are just holding things together, not being loaded in shear or tension.
I've heard that 2x4's are less likely to warp and twist than 4x4s if you must use 4x4s consider a pair of 2x4s side by side instead, or better yet use a 2x6, it will be much stronger than a 4x4.
Wood has grain, you have to take that into consideration when designing and building a stand, end grain is not very good for holding fasteners, end grain is not good for gluing, wood will split in the direction of the grain, wood swells more across the grain than along the length (a 2x4 will swell more in width than in length). Plywood adresses some of these issues by combining several layers of wood with the grain running in different directions, plywood does not hold fasteners well when driven into the edge, it may be worse than trying to drive fasteners into the end grain of solid wood. A superior design uses both plywood and solid wood strategically to take advantages of the benefits of each.
People like to claim they have over engineered their stand design when in fact they haven't engineered a thing, they just built something heavy, expensive and unlikely to break because of the amount of material in the stand. An engineered stand looks a lot like what some people are uncomfortable with. Good construction, quality materials and a sound design will hold a tremendous amount of weight.
Any guesses how thick the shaft of a screwdriver needs to be to resist breaking under the force of a human wrist turning a screw? It's something like the diameter of a thin coat hanger, obviously they are made to withstand abuse like being used as a pry bar, a chisel, a hammer, a lawn dart, or whatever uses people can come up with. This is a case of overengineering, because the tool has a potential for uses aside from the intended use it was designed to be much stronger than it needs to be, I can't think of many ways a tank stand could be misused, overengineering is really not necessary.
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