Found this an interesting read

[Evilweevil]

So after some searching all I could find was people asking if there tank would fall thru the floor not actual happenings off this , but i do find lots of stories of the seems splitting and the tanks rupturing ,so it makes me wonder if this could be what happens the tank gives way before the floor ever does .
Fill up a90 gallon with the back legs on a more solid joist the front feels solid but over time it flexes and gives maybe 1/8 to a1/4 inch the tank takes the stress and leads to failure in the tank ? I would think a tank could handle an eight but a 1/4 or more would be a lot

Might be a good myth busters episode

[Dearth]

I have found several instances of tanks falling through the plywood substrate but have only found one case of actual joist failure that was attributed to termite damage. A 110 gallon tank fell from 2nd floor through first floor to basement in a house in Georgia in the mid '90's and root cause was attributed to termite damage.