I am going to be setting up a 360+ gallon acrylic tank 96" x 30" x 29". The overflow's are on the right side and in the middle of the 29". Using this thread as a guide this is what I have came up with. My question is should the end support be supported from the bottom as well? What would you do different? The entire thing will be covered in plywood as well.
I think the design is sound except for the right end, and the number of cross members.
You've got a single horizontal piece (27" long" (28.5" long if you use 1x4's)) a few inches from the end, but it can't support much weight because it doesn't have any legs under its ends. You've got legs under the two little horizontal piecs at the end, but they don't span the whole 27" gap. What you need is a 27" long horizontal piece that spans the whole gap, and has legs under each of its ends. Since this is an acrylic tank, with a flat bottom (no raised rim) It doesn't matter much if that 27" long beam is a few inches from the end, or right at the end, but it needs a leg under each end of it.
The bottom of an acrylic tank wants to sag, especially if it's 30" wide, so you need to make sure that the top of your stand won't sag. Plywood sags, so the cross members will need to be able to support some weight, and be around 16" apart or less. With only two cross members, you'll have spans between them of around 28", so I recommend that you put five cross members between the two end beams, instead of just two, which will leave about 14.25" between them (or 15.125" if you use 1x4's). The end of each cross member must be joined well to the inside of the 96" long beams because each of the five cross member will be supporting about 400 lbs (some could support more if weight isn't evenly distributed). I suggest you use 2x3's or 2x4's for the cross members (even if you use 1x4's for the rest of the stand) and use galvanized steel joist hangers to attach the cross members. Some other types of joints, done properly, would work too, but I don't have the tools and know-how, so I go with joist hangers (just make sure they don't get wet and rust out).
You didn't say what size lumber you plan to use. I ran the numbers and according to the equation for deflection, since you'll never have an unsupported span longer than 28.75", you can use 1x4's or larger, or 2x3's or larger (Assuming that you can find straight lumber in those sizes, without splits or knots). You can use 2x4's if that's easier, but anything larger than that is overkill. I'd either go with 1x4's or 2x4's (2x3's work, but with 1x4's or 2x4's there's more room for building errors). 1x4's resist deflection much better than 2x3's, so you aren't cutting it close if you use 1x4's.
it will snap clean off at the wood 90% of the time i would use 1 screw to suck metal clips and brackets together and finish it with 1 1/4" structrual nails
