Ok so on a few different posts here people are talking about adding support under your floors to support your tank, which makes perfect sense.  This leads me to a couple questions. . .

At what tank size do you start doing this?

What kind of added support do you recommend in a small dirt floor crawlspace?

I think there are too many variables to give a one size fits all answer to this.  Things like how old is your house, how are the floor joists spaced, what size tank do you want relative to those variables,etc., will give the best answer for your situation.   Since the tank and location you are thinking about is giving you reason to ask such a question, I would have a structural engineer look things over.  Those services would be much cheaper than  the cost of a tank falling through the floor. Others may disagree.

You can help your situation by using a stand that distributes the tank weight over the full area of the stand versus one that just has four legs. You should set up your tank perpendicular to the floor joists (assuming you can figure out the directions of the joists in the first place).

If the aquariums weigh more than two large people standing where the legs are, I'd start paying attention to the floor- at least take a look at it. Say, two 300-lb people = 75 gallons of water, not even figuring in any stand, equipment, tank, rocks, etc. And a static load is going to require more support than someone  walking around. If your tank is near a load-bearing wall it will help somewhat.

New/old may not make a huge difference- some of the older houses are overbuilt, and developers of new homes do cut corners sometimes. They just go out of  business before the lawsuits start.

It depends on numerous factors.  Does the tank run perpendicular or parrallel to the joists?  How close is it to a load bearing foundation or support column.  What is the spacing between floor joists?  How big is the joist?  Is it engineered or just a normal douglas fir board?

Wood 2x10" and 2x12" can hold a lot of weight.  It just depends on the situation.  I would definetly want to place a bigger tank (say 180 +) perpendicular to the joists and as close to a load bearing foundation as possible.

The tank is 5.5' x 1.5' x 2'H, which figures out to be 125gal.  I'll be using canister's on it so really no xtra weight there.  The stand is wood and distributes the weight.  I haven't been under the house to see what way the joists run yet.

in my case i never did support for my 75s

but with 180 in same room i felt the need to support

my joists are doubled and 16 on center but with ground settling wood giving over the years i thought it would be wise btw my house was built in 72

if all this helps you

Rock