How long is your tank? If it's divisible by 16", which is standard spacing for floor joists, you should be able to position the legs over the joists and cracking the hardwood floor won't be an issue. I would be more concerned with the floor cracking as this could cause the cracked side to drop rather quickly, creating one heck of a mess.
If it's not divisible by 16" you could sister up the joists with a length of 2x6 & some carrage bolts to get a few inches, or nail a piece perpendicular to support a leg that is more towards the center of 2 joists. This is assuming you have access to the joists from underneath.
If you don"t have access to the joists get some scraps of steel plate to put under the legs large enough to span a few of the hardwood boards. Put these on the floor first and shim between the plates and stand legs. This will spread out the weight and decrease the pounds per square inch on such a small area. My 55 is on a metal stand, probably weighing 600 pounds total. If each leg is one inch square that equals 150 lbs. per square inch under the legs. If I put a 4"x6" steel plate under each leg the pressure is spread out over 24 square inches making it around 6 pounds per square inch. We use 1/2" plate at work to spread out weight on machines that weigh many tons. Even concrete will only take so much before it gives, and the plates do the job.
I wouldn't worry about the legs indenting the hardwood with shims under them. The shims are softer than the hardwood floor and would spread the weight more as well as being more prone to mark when weight is placed on them.
Tolak