I have one that's holding a 120 gallon perfecto tank... 60" x 18", so maybe it's the same tank.
I used 2x4s as the legs, 5 of them. One in each corner, and one in the back center. I don't think you'll run into trouble doing the same. If you are concerned about the strength, then I'd use a second 2x4 in between the top and bottom perimeter frames, glued and bolted to the first one. I put a link down lower to a different stand that I built, that shows detail on what I'm talking about.
For the bottom perimeter frame, 2x4s will be fine. Be sure that the legs are resting flat on the ground so the weight is on the floor, not on the fasteners.
The top frame I made from 2x6, with a doubled up 2x6 in the front to allow me to not have a center support.
For fasteners, I used liquid nails on all joints as well as a combination of other methods. On all butt joints for the top and bottom frame, I used 3" deck screws. To secure the legs to the top and bottom frames, I used 1/2" galvanized carriage bolts (2 on each leg at the top, one on the bottom).
This stand I did not document the construction of, but another stand I built I did, it's on my website
here. Hopefully, you can get some ideas from this.
BTW, if you have access to a table saw or jointer, it would behoove you to rip about 1/8" off the 2x4s or 2x6s that form your top and bottom frame along the top edge (top frame) and bottom edge (bottom frame) so that the surfaces that sit on the floor and that the tank rests on are perfectly smooth and flat. It helps tremendously to make a really nice solid stand. I didn't do this on the first few stands I built and now I do it on all of them, and it really makes a difference in giving the tank a nice true surface to rest on. If you don't have one of these tools, do NOT try to do it with a circular saw, etc... you'll never get a true enough cut, and end up making it worse.
