Might be a dumb question, but where does treated lumber fall in all this? Is it toxic or hard to stain, easily warped, or something? I have a chance to get all the stuff I want, for free, but if it's no good structurally or then I'll pass.
Treated lumber would really be any stronger or weaker, but it does contain chemicals. I do not prefer to use treated wood, but I've seen it used plenty of times. Just make sure you have a mask on when cutting it.
I'll speak from my engineering background on this subject.
Wood is going to be very good in compression. This is not what we're worried about here. If you can put a perfect compressive load on a single 2X4, it'll hold your whole tank.
Try the trick of standing on an empty coke can. It'll hold you up as long as you stand perfectly over the top of it. But you'll crush it if there is anything other than compressive loads on it (think of one of your upright 2X4's not being absolutely perfectly loaded).
Ideally, we should look at building codes to find what is "acceptable" under building code. There are tables for construction that will tell you the maximum span given a maximum load for types of boards as well as tables to tell you how much compressive load a given type of post can hold up.
I found using liquid nails are as strong as screws, practically indestructible. I have one stand which is more like a work bench now with only liquid screws. I once tried to break it down but was impossible.
Yeah, I used to build custom car audio boxes. Learned a lot about the best ways to connect pieces of lumbar together.
Screwing and glueing is pretty darn good (MUCH better than screws alone). I prefer dap, loctite, or elmers wood glues for this. I've seen a lot of experiments where these have shown to be the strongest for bonding wood.
One thing to remember when screwing and glueing is that the screws are really just there to clamp the wood together while the glue cures. Once cured, the glue is what's holding the wood together. But I do like having the screws there just for an extra peace of mind.
You can also just clamp the wood and get the same effect.
The problem with glue is if you make any mistakes,you can't undue it. I've had some vertical posts not perfectly straight and they did not fit well and didn't find this out until after the glue dried. Apparently placing a piece of wood upright does not necessarily mean it's vertically squared, next time I'll use a square to confirm each leg is perfectly straight.