I am more bold, so I do not sit on the furniture to test it, I stand on it. I only weigh about 145 pounds. But, if I stand on the surface and then move around it, I am putting a lot of psi on that spot. All of my weight is on one or both of my feet. However, the weight of a tank is spread around the bottom frame or the entire bottom glass for a rimless tank.
So, I know that of the furniture supports me, it should be able support a greater total weight spread out. However, there is a limit to the max. weight that would be safe. A 20 gal tanks empty weighs 25 pounds, a 20L weighs the same. This is according to Aqueon (which used to be All-Glass)
I have a dresser and a credenze both made or particle board covered in melamine. The credenza has held a 20L and then a 29 for decades. On 1/2 of the drrsser when I need an extra tank I set up a 20 gal. But I have large piece of 1/4 inch thick glass I put under the tank, The glass adds about 15 pounds.
Colin_T is right about older furniture. It was mostly made much sturdier than some of the less pricey stuff sold these days.
That is a nivce looking desk. I would use it as such and get myself a metal stand. I even make my own wooden stands out of 2x4s and sometimes a bit of plywood.
That's a good tip though, I'll have my friend sit on it with me.
