Aha, or a Waterbed with 2 people, guy is 250lbs and woman is 170lbs which covers the weight of glass/stand. Not to mention when they are humping. A fish tank is hardly rocked ever.
but a water bed + humping couple is spread (er...lol) over a very large surface area. often the frame has more than 4 contact points as well.
a 300g tank is 30" tall roughly so the weight is much more concentrated and places much more stress on the floor.