If you go with a large tank, anything over 125 gallons you should really look at the over all weight. At a little over 8 lbs per gallon, a 300 gal tank will weigh over 2500 lbs with stand and all.
I happen to be a floor joist engineer...... If your joists are running perpindicular to the tank, better, if they are running parallel, you could be in for trouble. If the tank is in the middle of a room you could bend the joist badly. if the tank is near a wall you could shear off a joist or two (or three!) worse case scenario is in a corner where the tank sits mostly on 1 or two joists, it will bend it and break it off at the end.
Most floors are designed to carry 40lbs psf live load, this is to say, load that is dynamic, it comes and goes. Fish tank weight stays, it is a dead load and it greatly exceeds the normal 10 to 15 lbs psf of dead weight that is assumed in most floor joist designs.
Don't get too excited, hot tubs go in a lot, and they weigh a lot too, but if you do the math you will notice by the time you figure in the foot print of the tub and the weight of the water, the hot tub usually weighs in around 40 to 50 lbs psf. And, the weight is usually live, meaning the tub is drained and stands empty a lot. When we designs floors with hot tubs, we consider the loads and design the floor accordingly. People who throw in hot tubs later, get what they get. problems don't happen that much.
Another thing with fish tanks, they are near water. better stated, the floor is near water. Everytime you drop a few gallons of water you soak your floor sheathing, and perhaps the floor joists. This can drastically reduce the load carrying capacity. You come home one night to find 20 gallons of water has siphoned off the tank and you worry about the carpet........ could be the least of your worries. Hopefully you are not renting, with renters below?
I would get some feed back from a structural engineer if you are considering a 300 gallon tank on a residential floor joist system. There are things you can do to beef up the floor, things to help spread the load around. Mostly, help pick a good place outright and avoid putting it in the weekest place right off the start.
Concrete basement floor? forget everything I just said.
If you happen to have a set a plans, or the ability to accurately describe the floor system, I might be able to help avoid a major problem.