I didn't realize glue was that strong.. An your talking standard wood glue ? Ya I'm prolly gonna go overkill on my build but I always want the best I want it to hold for long time. Next summer I will be building a 1000 to 1500 range size tank so I'm planning now an want everything right.
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Basic wood glue joints are very strong when gluing up wood face grain to face grain, face grain to edge grain or edge grain to edge grain. Once end grain is involved a standard glue joint is not very strong.
How this applies to plywood is a bit tricky since the veneers are mostly rotorary sawn and because grain direction alternates between layers. The upshot being that when joining plywood in the typical 90* edge joint as typically used in plywood aquaria, standard wood glue does not provide a very strong joint. At least not as strong as in the typical wood glue-up.
Also, glue joints rely on a near perfect junction between adjoining pieces of wood. When dealing with typical plywood and dimensional lumber you rarely get this so one needs an adhesive that has gap filling properties. So construction adhesive, polyurethane glue if the gaps are minimal or maybe even thickened epoxy.
My point being is that in the typical plywood tank there would not be much of a need for standard wood glue. I would use epoxy at the plywood to plywood joints since I would personally choose that for sealing the tank as well.
If my tank required external framing I'd likely use some sort of contruction adhesive for the junction between lumber and plywood.
Now, onto fasteners. For attaching plywood to plywood, I would use stainless wood screws. I don't believe there are lag bolts small enough in diameter for this and obviously bolts and washers wouldn't work here. I might also consider pocket screws.
For attaching the plywood to the framing, I would use either pocket holes in the lumber so there would be no screw holes inside of the tank or I'd use counter-sunk wood screws inside so there would be minimal patching.
The only place lag bolts or bolts and washers would be useful would be joining the external framing at the corners. And in this case I think I'd just use lag bolts over washers.
I suppose once could also use lag bolts to attach vertical framing members to the bottom framing plate since this is where some of the greatest stresses are on the tank. You'd have to take care to pre-drill with a large enough bit and to counter sink the heads.
