Structural Engineer here, hopefully I can give you some useful feedback.
A lot of your "how much can this hold" questions depend highly on the type of steel you are using. For standard structural tube shapes, the material is usually A500 Grade C. In terms of tube size, a single 2x2x1/8" of this material with a height of roughly 3 ft can withstand a vertical load of nearly 21,400 lbs (at 4ft you're at about 18,800 lbs). The 1.5x1.5X1/8" actually isn't a "standard shape" for structural steel, although you may be able to find it in home depot. The capacity will still be well over what you will need to hold the tank though. But the moral of the story is that the vertical capacity of the member isn't usually what determines if the stand will be strong enough.
The much more important aspect is the lateral stability. Think of bumping up against the stand, or more extreme, shaking during an earthquake. To achieve the lateral stability there are a couple of different ways you can go about it. Option 1 would be a "braced frame" which is essentially what you have in your first picture in the thread. With a braced frame, you have diagonal members that transfer lateral loads at the top of the frame into vertical loads at the bottom. This technique is usually used when you bolted connections (that can rotate) as opposed to welded connections.
If you have the ability to weld, the welded connection forms what is referred to as a "moment connection" Since welded connections are unable to rotate, the lateral loads are transferred to the base via the vertical members bending. (in the brace frame model, the vertical members only carry vertical/axial load, and the diagonal members carry the lateral load.) If you have the ability to get QUALITY welds, then I'd go that route as it's just less work. However if your welding skills are non existent, or you are bolting the tubes together, then go with the braced model.
Because of the nature of how braces work, the braced material actually can be pretty minimal relative to the columns/beams. If you're unable to do either, you can use the diaphragm method, and actually fasten a large sheet of metal/plywood to the outside edges of the frame, and they will act similarly to how the brace system works. This is also useful if you want the cabinet enclosed, as it kills two birds with 1 stone.
Last but not least, you also need to consider the bending capacity of the top span beams (where the tank is going to be sitting.) Using the same material we talked about earlier, a 2x2x1/8" tube a 4 ft long can carry about 1000 lbs/ft, which again indicates that you really don't have much to worry about regarding capacity of those elements.
The biggest thing is the lateral stability that I discussed before. Let me know if you need more info.
WoW so much info thanks alot. <3
I will be odering the stand from steel manufacturer (as i don't have the welders and skill
) so the welds i hope will be good..
All inside welds will be left as is for more strength.. All outside welds will be smooth..
The steel has these letters and numbers (S235) if helps?
The max shaking will be from train going by my house
You can see water shaking in the "water bottle" on my desk.
Let's say i don't want any "braces"(that you mention i had in my first post..) Can i go simply lagger diameter tubbing to increse stability? Example 2.36 inch tubes? Still 1/8 thick, will that work out for simple 4 legs?
Overall the max weight will be only 55-65 gallons. I think that would be so strong.
(Also less material and weight from stand with 2.36 inch tubes)