Just wanted to pop in here an provide a different viewpoint. I am going through some struggles with placing my tank (RSR350, under 100g total) on a second story type scenario. My home was built in the 70's and I have 2x10's at 12". I sistered those joists under the tank and near the tank after consulting a structural engineer and for peace of mind. Well, it wasn't peace of mind. Not because the joists couldn't handle the load but because my structure is settling very slightly on the hillside. This has caused small leveling issues with my tank that have slowly warped the stand every so slightly, but enough to cause a bubble in the seam of my rimless tank. Now, this bubble could be caused by manufacturing flaws, but it does appear near a potential high point in the stand. My community conducts surveys of the homes every year and the surveyors did find a small amount of movement -- so I have some level of empirical evidence.
Anyways, I'm not trying not bring my problems into this thread or scare you off, I just want to point out that you should consider more than just whether the joists can handle the load: if your building is shifting or settling, the condition of the joists, what kind of wood, etc will all have an effect on the capacity of the structure. I doubt you have to worry about your tank crashing through the floor; the more pressing issue (especially with a glass and double especially with rimless) is the leveling issues that may arise over time due to settling or other factors that may torque, twist, or tilt your stand in ways that would compromise the tank's integrity. Again not trying to scare you, just giving another point of view. What I'm saying may not apply at all. You can always consult a structural engineer!
