Exactly what I was going to say, and I couldnt of put it any better myself.r1dermon said:ok, two theories posted just dont work. the plywood....reason: plywood is QUITE flimsy. take a sheet off of the rack at home depot and try to bend it. it will bend with ease. therefore, it doesnt distribute the weight very good, and as such, it just adds more weight to your floor and looks really bad. a better alternative to this is to make a box-type platform with 2X4's spanning to 4x4's under a sheet of ply or particle board. this will distribute the weight MUCH better.
also, the people standing on top of the stand thing. that doesnt work because, thats a significant load for a short period of time. if you are wondering if a certain piece of wood will hold you, how would you test it? you would jump right? well, that is a lot of load for a SHORT duration, and as such, it may not break under that load, rather, it would break after a couple days or weeks of continuous exposure to that load. the people standing on the stand would be an example of that.
here's a significant point of interest. just because your floor doesnt "break" doesnt mean that it is not damaged. if the load is heavy enough, without being OVERLY heavy, it will gradually bend the joists under the floor. and in a couple years, ten even, the floor joists will snap at the center, your floor will bow in the middle, or the joists doing the supporting will all be off-centered because they've got so much pressure on them.
some apartments this is not a problem for, either because they used steel beams for construction instead of wood joists, or because its concrete. either way, that type of structure could easily handle a 300 gallon tank. if you have doubt about it, do not do it. assumption is the root of all failure.