I'm assuming these are glass tanks...and that there are a dozen different theories on this, differentiated according to tank size, framed or frameless, inside-bottom or outside-bottom, soft gasketing or not, etc.
Don't know, don't care what the theories say. My first actual fish room had three racks, each supporting 19 tanks on three levels, all 10's, 15's and 20's with frames. Every one of those tanks simply straddled the front and rear horizontal beams of the rack. The 10's were supported along the entire length of their ends (short sides) only. The larger ones sat the same way, except that they extended past the rack on the front and rear and were supported only on 4 contact points, 2 on each long side. All the tanks were viewed from their short ends. So, only the 10's had support at all four corners and along their short sides. The others had no corner support at all, and a total contact length of about 6 inches (4 points multiplied by the width of a 2x4, i.e. roughly 1.5 inches). A thin rubber strip, cut out of rubber floor mats, padded each contact point.
I'm sure I was voiding warranties, risking life and limb, blah, blah, blah.
The set-up lasted for 7 years, without ever developing a leak in any of those tanks. I had a couple of leaks elsewhere, including one catastrophic failure of a large 100+ gallon tank; all those problems occurred in tanks that were on commercial stands.
Full disclosure here: back then the glass used in the construction of larger tanks...50 gallons and up...was thicker than is commonly used today. I don't know if that was true of those smaller tanks as well, but if it was that would have been an advantage. Also, the floor those racks stood upon was a rock-solid, perfectly-level, steel-reinforced slab...not the wavy, wobbly, wabbly floor seen on the floor of many homes today. And finally, the racks were built under the guidance of my father who was a very talented DIY guy and an absolute stickler for careful measurement and construction; those racks were works of art, not pretty but dimensionally perfect.
