1/2 inch glass vs 3/8 inch glass

you could always brace the floor from underneth house but i wouldnt think the new tank would make your floor collaspe tho but if your floor is iffy now i would suggest bracing the floor.

The weight of the aquariums empty (3/8 vs 1/2") may feel significant but when they are full of water the difference in the weight of the glass will be negligable. My thoughts are, if the tank looks nice (glass is in good condition, doesnt leak, no scratches, etc) just go ahead and use it.

Some miscellaneous thoughts. First of all, it is not possible to determine the structural integrity of your floor by looking at a picture of the surface. The most important factors are all underneath the floor.

Second, if your calculations for the volume and weight of the full aquarium are correct then consider this. It is about 640 pounds spread over 6 sq. ft (4 ft x 1.5 ft) which yields a pressure of about 107 pounds/sq ft. Now, my own feet are approximately 12" long. The area of both of my feet is about .8 sq feet. I weigh about 185 pounds, which yields a pressure on the same floor that your aquarium would sit on of 231 pounds per square foot. Your larger aquarium yields a pressure of 1218/8= 152lbs/ sq ft (I am assuming that your calculations are correct). 152 lbs/sq ft is still less then the pressure that I would exert on your floor. That is one way to think of it. You can make the same calculations with your own weight - you will probably find that you exert more pressure on the floor per sq ft. than the aquarium will. Now, the big question is, will the floor hold the pressure over time?

when houses are designed the weights are calculated by how much the whole room will hold not just the weight applied over the surface area of a certain object, the weight of that tank will be applied across the whole floor, i think you'll be safe but here's a link to a site that explains a lot, personally i wouldn't worry about it. think of it this way, a woman weighing 100 lbs wearing heels is putting about 1200lbs per square foot on the spot her heels are and she doesn't hurt the floor, so don't figure it by how much weight per square foot, like i said i wouldn't worry about it at all.

http://badmanstropicalfish.com/articles/article28.html

I agree if its a stick home and not a mobile home you should be fine but not as fine as a concrete slab foundation obviously.

But here is what concerns me your aquarium is supported on a steel frame which is strong enough but all the weight is being transmitted to what looks like four 1/2 inch legs, which means that the steel feet are exerting allot of force on your nice hardwood floors perhaps to the point that you will have indents in the wood. I recommend you place something under the feet to displace the pressure to protect the finish especially with a larger aquarium if your using the same stand. A cheap solution would be 4 blocks of wood 2x4 or 3/4 inch plywood pads, or a sheet of 1/2 plywood that fits under the entire 4 feet base.

Looks like the feet on that aquarium are good to go since they have pads.

How convenient is it for you to clean out your sump? Do you have to remove it or just scoop out and replace media. Looks like your sump is almost as big as your tank, supposedly a plus. I personally wouldn't know as I don't have a sump (yet), so I'm just curious as to how its worked out for you.