So I'm planning on making a stand but using only 2x4's. My tank is a Mini-L with the dimensions: 45cmx27cmx30cm (approx:18"x10.5"x12"). My question is: without adding a plywood top, would the tank be safely supported with just the outer edges of the stand? Also, what's the highest I can safely make the stand if the stand were to be made close to 18"W x 10.5"D?
For a tank of that dimensions, you could do a really simple (and easy) stand out of a couple lengths of 2x12 - it will be wide enough(~11.5") to support the width of the tank, and it's plenty strong. I've currently got a 20XH set up on a stand made out of a single 2"x12"x12' (meant to get 10', but wasn't paying attention, and have a bit over a foot of scrap 2x12...). I'm trying to upload a pic from my phone, but sometimes it takes a while (or just doesn't work...)
basically take two pieces for the verticle segments, and run some simple dados in them about 3/4-1" deep(I did mine with a circ saw and chisels), and then stick the shelve segments in them, glue and screw, finish, and you're set. -I've got a set of bookshelves about a decade old I did in the same manner, and one of the ends is cupping, but the horizontals have been held pretty stable by the dados. And it's pretty strong, especially considered the time and planning (or lack thereof) that went into it.
aaand... I can't get the pic from my phone (and likely you wouldn't be able to make out anything aside from the clutter on/around my stand anyways...), so here's some google search images to try and demonstrate:
like this, except rotated 90 degrees
just like this, except only two shelves, and actual-sized
I hope some of that made sense, it really is easy, and strong...